A few things first. "32 bit", "16 bit" and "8 bit" refers to the colors used. I don't remember the numbers exactly, but that means around "millions of colors", "256 colors" and "64" colors respectively. Although this is also limited by the hardware in hand. The NES for example can't display all 64 colors of its palette at once, even though it's an 8-bit system
As far as dimensions go, RPGs are tiled and so you'll want to use one of the default sizes like 16x16, 24x24, 32x32 or 48x48. If you need visual idea of what those look like, look up the different RPG Maker editions as they come with a pack of graphics in those sizes. RPGM 2k3 uses 16x16, RPGM VX uses 24x24, RPGM XP uses 32x32 and RPGM MV uses 48x48
You also need to pick
a resolution for your game, which should compliment your tile dimensions. You can make a game 256x240 if you use 16x16 tiles, but you need to go bigger if you want more detail
Since you're new, I recommend that you stick to the smallest resolution possible. Pixel art gets exponentially more difficult as you increase the size of your canvas. Making a 16x16 tile may take a couple minutes, but a 48x48 tile can take hours and a much higher set of skills
Here's a few links to help you out:
This is a great basic pixel art tutorial that may help you getting startedA little guide on perspectives