Answer:
Sizes are important (and annoying, if you ask me) to heading out on missions. There is a size limit in general, which is 8 stars (I think). You can find what size a Pokemon is by reading it in its summary. Some missions will require only a certain total size of Pokemon can go on a mission.
I think you can assume this, but I just wanna say it: Some Pokemon are so colosal, you can only send yourself, one partner, and said Pokemon on a mission. *cough*Dialga*cough*