I used to be with Sprint; the last phone I had was the Touch Diamond (HTC). It's about 4" tall, 2" wide & a half inch thick. And it's light; weight 4oz.
I was able to get the phone activated on boost. I don't know if it's the smallest phone but I like it. Works perfectly.
I haven't tried this, but I've got an old touch diamond as well. I can't imagine finding a phone that is much smaller.
If I understand this correctly, I can just hard reset the phone to clear out any old junk and then I should be ready to go?
What is a "clean" esn? I assume since I've only used my phone when I was on contract with Sprint that I have one.