As Phoebus say a 16 tileset would be best
or 16x16
That would fit full screen perfectly for you.
Might seem big or a little cut off on windowed mode though.. If you need help with the init settings I could throw something together real quick.
16x16 is the smallest tileset I think you'll get with "graphics"
To add on:
Though there have been SOME attempts in the past (check old wiki page on gfx tileset), the gfx are usually so small that personally I feel that they are not very usable as gfx. Anything smaller than 16x16, I would really recommend running in non-gfx mode. It is a little intimidating at first, but after you get used to it, I think it is actually better than small hard-to-distinguish/recognize gfx tiles unless you can read information off "colored-barcodes".
My personal 3 guidelines to using gfx tiles (if not met, should reconsider using gfx at all):
1. Tiles should be distinguishable (And not just being different, e.g. 1 pixel in different color in a complex pattern).
2. Tiles should be intuitive recognizable (e.g. is "E" an Elephant or an Elf? definite not for beginners. gfx tiles can solve this, but only if it is STILL recognizable.)
3. Aesthetics: Using it should make your game look better and easier to comprehend. If using gfx mode makes things more confusing or cause you to bleed your eyes singling out details: please consider using the original ascii mode. really.