Does size affect the pokemon in pokemon go?

Not in any way that anyone has been able to determine in a year and a half. It is most likely just flavor text.

Only for collecting your "Fisherman" and "Youngster" badges.

It will when breeding comes ;)

"Does size affect the pokemon in pokemon go?"

Ask your girlfriend about that, ....

(Sorry, couldn't resist.)

I've heard once that XL and XS size are important. XL are usually best defenders, because they're bigger and have higher PS and Defense. XS are smaller and best attackers. They're usually faster and have higher attack stats.

I really don't know if it's really real, but I'm gonna try it.