Player size != tile size
Speaking about tile size I join @Pharap, 12x12 seems the right spot given the resolution. However the drawback is that well… Most existing tilesets are 8x8 or 16x16 so it will be harder to find references, or “get inspired”. At the end 8x8 will be the safest for low-rez, and 16x16 the safest for high-rez.
For the player, in CastleBoy player is two tiles high and can jump two tiles high. I did a presentation about CastleBoy at my local gamedev group, the slides about design could give you some insight. You can skip the first part it’s about the Arduboy
CastleBoy_fnt.pptx (1.2 MB)
Note that the fact the player is 2 tiles high and can jump two tiles was due to the Arduboy’s resolution which is very wide. While doing the level design I noticed that it would have been much more interesting to have a player that is 3 tiles high, yet can still jump only two tiles. I do believe this is how Super Castlevania IV works (on the SNES). I can only recommend playing this version of Castlevania, it seems to be more less what could work on the Pokitto. Moreover it’s an amazing game.
For the little story, the first Castlevania on GameBoy is the first video game I owned (with Tetris) so it is very special to me. This has been the main inspiration for CastleBoy, even if this particular version of Castlevania kind of sucks.