I am trying to figure out how I could homebrew something that will make the PCs tiny, reduce their speed to about 10 ft, reduce their STR.  When they actually equip this item, I would like their sheets to adjust.  Any suggestions?

If it’s possible then this is how:

Thanks!  I had two of those but for some reason they were not showing up on the character sheet when I equipped the character.  Had to delete from the inventory and then re-add it back several times.
 

 The Size - Tiny Modifier doesn't have any effect at this time, but it's still worth having.

You may also want to add the modifier Carrying Capacity - Tiny to give that extra reminder of tininess to go with the strength & speed reduction.

This might be a helpful reference:  (credit where it's due: this is paired down from a post by  Chicken_Champ) I've been playing a Fairy so I had the size rules handy.

Player's Handbook, Chapter 2, Racial Traits: 
*snip*. Members of a few races are Small (between 2 and 4 feet tall), which means that certain rules of the game affect them differently. The most important of these rules is that Small characters have trouble wielding heavy weapons, as explained in chapter 5.  <-Tiny: under 2 foot

Player's Handbook, Chapter 5,
Weapon Properties:  Heavy. Creatures that are Small or Tiny have disadvantage on attack rolls with heavy weapons. A heavy weapon's size and bulk make it too large for a Small or Tiny creature to use effectively.

Player's Handbook, Chapter 7,
Strength- Carrying Capacity: Size and Strength. Larger creatures can bear more weight, whereas Tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag, or lift. For a Tiny creature, halve these weights.

Player's Handbook, Chapter 9, Movement and Position: 
You can move through a nonhostile creature's space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you.

Player's Handbook, Chapter 9,
Grappling (and shoving too, just below it):  The target of your grapple must be no more than one size larger than you and must be within your reach.

Player's Handbook, Chapter 9, Mounted Combat:
A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount, using the following rules... 

The size change can make animal companions that would normally be too small to serve as mounts (small and medium) suitable mounts.

Monster Manual, Introduction,
Statistics: A monster can be Tiny, Small, Medium, Large, Huge, or Gargantuan. The Size Categories table shows how much space a creature of a particular size controls in combat. See the Player’s Handbook for more information on creature size and space.

Size              Space              Examples
Tiny              2½ by 2½ ft.   Imp, sprite
*snip*

Dungeon Master's Guide, Chapter 8, Using Miniatures:  
Creature Size on Squares and Hexes A creature’s size determines how much space it occupies on squares or hexes, as shown in the Creature Size and Space table. If the miniature you use for a monster takes up an amount of space different from what’s on the table, that’s fine, but treat the monster as its official size for all other rules. For example, you might use a miniature that has a Large base to represent a Huge giant. That giant takes up less space on the battlefield than its size suggests, but it is still Huge for the purposes of rules like grappling.

Size       Space: Squares     Space: Hexes
Tiny       4 per square          4 per hex
*snip*

Last edited by Chicken_Champ: Nov 4, 2020

Good info! 

Is there anyway to a monster statblock so that it changes it auto changes it's str score and it's attack/damage modifier 

Quote from Rexs424 >>

Is there anyway to a monster statblock so that it changes it auto changes it's str score and it's attack/damage modifier 

 What do you mean?

In the campaign I am in I am something like a summoner  and when I summon a doll it's strength attack/damage and hp change depending on the dolls size I am asking if there is a away to do it auto so it auto changes it's stats based on it's current size

Quote from Rexs424 >>

In the campaign I am in I am something like a summoner  and when I summon a doll it's strength attack/damage and hp change depending on the dolls size I am asking if there is a away to do it auto so it auto changes it's stats based on it's current size

 
Sure. Create a different stat block for the monster for each size.

ok so i have to do it the complicated way dame that will be annoying what with me being able to fuse them to gather and make lots of them but thanks for trying to help