I do hope others chime in but I might go with the two gallon for a full years growth. Awesome you got your clematis! I know how frustrating it os to never see anything new and exciting.

I pot up into a gallon pot until fall or the next spring which works well for me.

It just occured to me I could do the gallon and if the roots start coming out the holes it can go to 2 gallon. Duh. Where's my brain? Thank you both.

(At least I am spared the 5 gallon route!)

Haha I was thinking the same but when you said you intended for them to stay there I thought two gallon . Some might not grow as much as others but I feel like your type 3s will grow fast! :)

I also plant up to a 1G if a small start (I.e. Costco bare root) but I plant out in fall. You do not need to go larger.

Lily, you may be right. This is Calif. with a year round growing cycle. I wanted to do the job once this year and be done with it untl next spring. No rest for the gardener here.