I'm an artist- but as far as game development, most of the games I've been interested in making? I'm limited to background art, concept art, and promotional art. But I want to make art for the games itself! I want to learn 3D, but we're not getting into that yet. Pixels. 

So I want to practice making a few pixel sprites and backgrounds. I am having a lot of trouble wrapping my head around tilesets, though. Does anyone have a recommended template, pixel size, or anything? And do you just almost make like a small picture within that tileset (say your game has 9x9 tiles) do I just try to think of things in that size? It'd take up two tiles, but not go all the way to the top, if I wanted my character to be taller? Or a tree thats taller? Do I generally keep things in that size? I want to practice by making some cool assets, and eventually make my own game. For now, though, I'll probably stick to sidescrolling perspective or top-down while I figure things out. 

Start with a larger canvas than you'll need, you can always crop out the excess space later. There is no uniform sprite size you should use, it's determined by the resolution of the game it will be used in. If you already have some tiles for scale then just make sure your character looks in proportion to them. You don't need to make the character size an exact multiple of the tile size. Everything just needs to be visually in proportion with eachother. If you create a tileset for a house, and then a character to go with it, as long as he looks like he'd fit through the door then his size is fine.

If you want a jumping off point I'd recommend looking at older games to see the sizes they used. For example the NES ran at 256x240 and used 8x8 or 8x16 sprite sizes (but lots of developers just put a bunch of those sprites together to make even taller/wider characters). The background tiles were 8x8. The SNES could have sprites up to 64x64. You're not limited by hardware the way these developers were, but if you want to make sprites that look authentic then I'd suggest using similar sizes.