Lets say you enlarge a Goliath that’s 8ft tall would it become a large or huge creature. Because the spell says it goes up one size category but the doubled height would be 16ft which would put it in the huge category or am I just overthinking it.

also would the enlarge stack with the rune knight’s ability to become large.

I’m pretty new to all this stuff 

Unfortunately height does not equate to size. The spell specifically states one size category. The size of a goliath states the following:

Goliaths are between 7 and 8 feet tall and weigh between 280 and 340 pounds. Your size is Medium.

So you go from medium to large.

Height and weight are pretty much fluff mechanics that get overruled by other game mechanics. A Goliath with 20 strength could carry 600 pounds, and because of powerful build drag, push or lift 1200 pounds. So you should be able to pick up a 40 pound halfling like a cup of tea. However grappling states:

Moving a Grappled Creature. When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.

So in effect the amount of weight you can push, drag or lift has no implications on actual combat mechanics or the like.

A 4-5 feet 150 pound dwarf and a 7-8 feet 28-340 goliath are both considered "medium" even though, compared to a small dwarf,  a tall goliath is double the size and weight.

Oh ok thanks, personally I think that’s a little dumb but oh well 

but here’s a thought could I use giant’s might and then be enlarged to become huge or gargantuan depending on the level of it

There's nothing stopping you from using the Rune Knight class feature and then using Enlarge/Reduce. It has to be in that particular order though, as Giant's Might doesn't care about your previous size, it just sets your size to large.

A Gnome could use giants might to first become large, and then Enlarge to become huge.

What might happen if I cast this on a thrown projectile already in motion? or is that a DM call?

Quote from Pancakeli >>

Or you could have several casters using the same spell on one target. So far as I know there is no rule in any book that says "A target cannot be under the effects of the same spell cast by multiple creatures." So if you had 2 people reduce one target than suddenly the target is a quarter their original size and a sixty-fourth their original weight or inversely 4 times as large and 64 times heavier. 
If you got 3 casters with Enlarge/reduce suddenly you have a (presumably originally medium) barbarian who is as large as the Tarrasque.
You could go 1 level of rogue for expertise in athletics, maybe pick up the grappler feat, and now you can have a nearly fair WWE match with dnd's very own Godzilla. 
EDIT:  So unfortunately after some reading of some rules, a target cannot be under the effects of the same spell multiple times, so unfortunately there can be no gargantuan barbarian wrestling match without some breaking of the laws of reality  :( (otherwise known as homebrew ;) )

 A level 18 Rune Knight Fighter can become huge with Runic Juggernaut and then get a caster use this spell on them and there you go, one Tarraque sized PC who can fist fighter ancient dragons

Now with the Rune Knight, you can!

You could use this spell in conjunction with a Potion of Giant Size and still get to Gargantuan.

One could also get a Potion of Growth and a Rune Knight could get to Huge. Or the Rune Knight could take a 3 levels in a class that can cast this and then be Huge several times a day.

This should have a note in the "Available For" field that Druids can take it if using the Tasha's options.

Is there a limit to the size of the objects that can be affected by the spell? For example... a navy ship or a galleon? Would a galleon count as a structure and thus be immune to the spell?

Quote from Pancakeli >>

Or you could have several casters using the same spell on one target. So far as I know there is no rule in any book that says "A target cannot be under the effects of the same spell cast by multiple creatures." So if you had 2 people reduce one target than suddenly the target is a quarter their original size and a sixty-fourth their original weight or inversely 4 times as large and 64 times heavier. 
If you got 3 casters with Enlarge/reduce suddenly you have a (presumably originally medium) barbarian who is as large as the Tarrasque.
You could go 1 level of rogue for expertise in athletics, maybe pick up the grappler feat, and now you can have a nearly fair WWE match with dnd's very own Godzilla. 
EDIT:  So unfortunately after some reading of some rules, a target cannot be under the effects of the same spell multiple times, so unfortunately there can be no gargantuan barbarian wrestling match without some breaking of the laws of reality  :( (otherwise known as homebrew ;) )

U can just go 3 levals in rune khiat  and 2 levals in wizard and for race go Dragonborn with tail get lighting breath or fire there U have Ur Godzilla or for race U can pick lizqrdfolk for Godzilla without atomic breth

When will this (and other spells) be added on the bards spell list? Seems to be a small fix, but Tasha's is out for a while already.

Slap it on an 18th level rune knight. WWE Faerun is back in business

Same for Druid's spell list.  I'd love to have my low level char turn into a huge Ox or Constrictor.

There’s an interesting loophole to be exploited here. If you stab someone with a dagger, then cast enlarge on the dagger, nothing will happen bc in terms of RAW, a dagger embedded in someone is, for all intents and purposes, being carried. However, if you cast the spell on a willing creature, there is no saving throw AND all of the creatures carried items get enlarged too. This specifically overrides the “not worn or carried” clause. So you can have your rogue pop up from hiding, stab the giant monster with a sword (preferably a large one) and keep holding the sword, then have your wizard cast enlarge on the rogue. + 1 giant rogue, + 1 giant dagger, - 1 monster, which has now been torn apart by an 8ft long sword that has just appeared inside its body.

So you have a item too big to fit into a bag of holding, cast reduce on it, it now fits in the bag of holding. What happens after you put a reduced item in the bag of holding? Will it expand in the extra dimensional space?

I think if someone tried that at my table, I would rule that it ruptures the bag of holding and would follow the bag of holdings own rules:

"If the bag is overloaded, pierced, or torn, it ruptures and is destroyed, and its contents are scattered in the Astral Plane."

So if your character uses Reduce on a character that weighs 80 lbs or less, you can combo it with mage hand to make them fly... And it says you have to see your target. That includes you, if I'm correct.

Flying without Fly.

Cast it on an ancient red dragon and see what happens.