I'm going to a reptile expo this Sunday and I'm super excited since I've never been to one before! I have a list of supplies that I'll be getting and at the top of that list is a second WTF

I plan on quarantining it for at least 30 days and I'm hoping for a female (although I know I probably wont know for sure). So I'm trying to figure out what a good size to get would be so that after 30 days or so it will be safe to to put in with Glurmo. He's about 2 inches give or take. I'm guessing he's about a year old so his growing has slowed. So I really have no idea how fast a juvenile would grow. I don't want to have two tanks set up forever! So what's the minimum size I should get? I know growth depends on conditions so just assuming ideal conditions. Also, any good tips for picking out a healthy frog?