You're a bit all over the mark here ;) It's tough because there are a lot of choices in your price range, starting from quite inexpensive (such as CmoyBB well south of $100), intermediate (such as C421 @ just under $200), the headstage arrow or alo national ($300), right up to the pico slim, TTVJ slim, etc.
Here's some good starting points for you:
Budget amps round-up: http://www.headfonia.com/music-on-a-budget-sub-100-portable-amps-shootout/
(includes CmoyBB, others)
A bunch of portables round-up: http://www.headfonia.com/the-usual-suspects-12-portable-amps-compared/
(includes pico slim, mustang, etc)
JDSLabs C-421: http://www.headfonia.com/?s=c421
Once you've read through all that you will be in a better place. I think if form-factor is your absolute most important thing then the pico slim, mustang, or headstage arrow will probably fit the bill nicely. If you're okay with a bit bigger but still quite slim, the C-421 is great, or even the E17, and both are quite a bit cheaper. The CmoyBB is supposed to be quite good for it's price, but is quite a bit bulkier and the whole mint-tin thing never really did it for me personally, but your mileage may vary!
Thanks just took a read through.
Right now I'm looking at the Fiio E17 and the Pico Slim. Not sure which to go with though. I heard the J3 doesn't need a DAC?