>>58627425Depends on the Pokemon! Some of them are still pretty big, like Donut here
>>58623740. XS Gardevoir is another one with a nice size though!
>>58627436I generally just eyeball it. To help with this, I have a couple boxes named Tiny Hunt that I keep small or large Pokemon in that I'm interested in going after. If I happen to see one that looks small, I'll pull out one I already caught to compare them. Always save beforehand since you might black out due to wild Pokemon; if you're not sure, may as well catch them anyway. Would be a huge pain to do with apricorn balls though.
If you mean how to spawn different sizes, it's the same as shiny hunting. Fast travel, bench, stairs/sewer entrance, walk far enough away, etc. The difference is that you have to actually go through and check all the spawns each time; you can't just AFK and wait for a shiny jingle.