Homebrew Race - Lineage races, like Dhampir, allow you to select your size. On Dhampir, the setting is found in Racial traits \ Size \ Modifier \ Size - Choose.
But in other HB race building, the "Choose a Character Size" option is not there.

(i noted this a few months ago.  if it has been corrected by now, okay.)

You create two Options, one named Small and one named Medium and attach the corresponding Modifier to each Option.

Thank you.

I can't find that option.

Quote from Halfwing >>

If you are creating a base race (not a subrace or a variant), you should see a drop-down called Size that’s in the top right corner of the Basic Information section of the homebrew form. This is where you’ll find that option:

I tested that and it doesn’t seem to give an option to choose your character’s size. If I recall it works when chosen as a Modifier Subtype (I think), but not when selected in the field you indicated. At least, i don’t think it did the last time I tested it.