16x16 tiles are fairly standard. We have used 8x8, 16x16, 32x32 and some other wierd sizes, so don't feel like you have to restrict it one way or another.
few quick comments:
- seems like you have chosen to make the few characters/enemies very very small. i would try doing larger character pieces if this is meant for portfolio
- all the images seem very monochromatic...like they all rely on similar colors. I would try to expand your color use
- making actual mockups is a good way to show you understand and can create all necessary elements of a game (tiles, characters, effects, HUD, etc)
you obviously have talent, i think you just need to focus on getting impact portfolio pieces that show potential employers what they are missing by not employing you. cheers.