Is Concert size easiest to learn on?
It just might be, yes.
Eh. I have a lot of sopranos, one concert and one tenor. I like the concert the least.
I must be the exception to the rule. I have large hands (XL or XXL gloves), a guitar background, and the soprano seems to me to be the easiest to learn on, unless you are playing way up the neck of the fretboard. I only recently picked up a concert, and it's been noticably more difficult.
Guess I'm just weird!