I have created a homebrew race where, unless specified by the subraces, the size is "Medium or Small." Whenever I create a character with a subrace that doesn't have a size set, there is no option to choose. How would I do that, or is this just a bug? I don't really want to create a whole racial trait just to be able to select size, because I can't find a snippet code where I can list the size. I feel like "Your size is either Medium or Small" still seems a little clunky.

The “Medium or Small” Modifier Subtype is for monster statblocks. In truth, the whole “Size” Modifier is for monster statblocks, but can be used on PC options. However, the size Modifier does nothing mechanically to a character sheet. What you want to do is create a racial trait named “Size” with two separate Options under that trait, one named “Small,” and the other named “Medium.” Then attach the following two Modifiers to each of those options: 

• Modifier: Size->Subtype: Small (or Medium)
• Modifier: Carrying Capacity->Subtype: Small (or Medium)
  (It should go without saying that you want to use the Modifier Subtypes that match the name of the Option those Modifiers are attached to.)

I hope that helps.