Hello, if anybody can help me with this, it'd be much appreciated. 

So I'm making a Homebrew Raccoon-folk race that can be either Medium or Small depending on the players choice, but I can't figure out how to add the option. 

I've set the race to "Medium or Small", but that only doesn't give the option to choose. Usually, when I want a Homebrew race to have a certain trait, but don't know how to add it to the builder so it actually shows up on the character sheet, I'll make a copy of an existing race that has the same or a similar trait and copy it from there. But when I did that with this (I made a copy of the Horengan, if that makes a difference), it said to add a Size modifier and to select " choose a size" or something like that, but that option doesn't exist in the builder. 

Is there something I'm missing or just doing wrong? 

Edit: Thank you, everyone. The video was extremely helpful, and I was able to fix it. 

As part of the Size trait you give it, you have to add the following Modifier:

• Modifier: Size->Subtype: Choose a Size

Yeah, I tried that, but it doesn't show the "Choose a Size" subtype. 

I think Sposta is misremembering as there is no such modifier type that we are able to use. Or at least none I can see. 

You will need to have size set up as Options. I made an unlisted video for you showing how. 

Quote from Cyb3rM1nd >>

I think Sposta is misremembering as there is no such modifier type that we are able to use. Or at least none I can see. 

 
That used to be a Modifier. I know because I used to do it with Options and then they added that Modifier and I didn’t have to anymore.

What’s that you say? Pictures or it didn’t happen you say?

Lemme ask ya something: Do you like apples?

How’ya like dem apples?

How do you have that when I don't? O.o

You can see in my video I don't have it.

It’s not available any more, but like I said, it used to be. That’s from a race I created while it was available. Try making a race using the Aasimar from Mordenkainen Presents. That race uses that Modifier. Or at least it did.

It's even weirder when you realize out that if you homebrew based off the 2024 Tiefling, it's an option, but not as one you can choose when starting from scratch. 

So, the video doesn't work for me (I'm late to the party, so I apologize), and I didn't see anyone actually answer the question otherwise. So, for all those future people who don't want to struggle through this (like me!) with the new ruleset. Here is how I found how to do this!

Basically, you need to create multiple traits with the same name. For my example, I will be doing the options of Small/Medium sizing and all traits will be called "Size".

1. The 1st Trait: This should be your "choose between" flavor text trait. The key to this trait is that you want to set the "Feature Type" to "Granted". Enter your flavor text (as usual) and save. DO NOT add any options or modifiers to this trait, leave it blank.
2. The 2nd Trait: This is your 1st "Option" (My case, it was Small size). I kept the same name ("Size") and set the Feature Type to "Replacement", selecting the original "Size" trait as the trait to be replaced. Fill in your new flavor text (e.g. "Small (3-4 feet)" for the snippet) and save. From there, this is where you add the modifier. The Replacement traits get your modifier, and for this case its 1 modifier per trait, as shown below:
   • Modifier Type > Size
   • Modifier Subtype > Small
3.  The 3rd Trait: This is your 2nd "Option". This trait, and any additional options, will be set up similarily to your 2nd trait. Feature Type = "Replacement", Trait to be replaced = "Size" (Original "granted" trait). Flavor text, save and add the 1 modifier:
   • Modifier Type > Size
   • Modifier Subtype > Medium

This is significantly more tedious than the original way of doing things (thanks WoTC!), but it works pretty much the same for the player/user. Just remember, the Replacement trait is your "Options" (so stupid, because the "Options" inside the trait don't do jack!).

Hope this helps!