I have had the same problems with changing sizes or systems
Isn't that the tool 'porn' side of organizing as David refers to it?
So, what I decided to do was go with the letter size since 99% of my printouts are letter size and I don't have to go reduce / cut etc.
I do have the junior / smaller sizes sitting in a drawer beckoning every time I open the drawers to file something - use me please. So far, I've gone 4 months and am ok. I'm sure there will be a time in the future where I will justify it in my mind to switch back to a smaller size, who knows for what reason at this point. For now, I am able to stick with the letter size.
Oh, I can see it now while sticking with the letter size - switching between ring binders and circa and vice versa.