Question about custom sprites

Depending on the source port, yes. You can replace the sprites in any source port, but the default resolution is fixed, so the size in game is dependent on the resolution of the sprite.

Modern source ports (such as the ZDoom family, probably others too) allow you to scale the sprite down in game, so you can add in higher resolution images. In ZDoom ports, this can be defined in the sprite's Decorate file. 

You should also read up on the sprite naming convention as well. It can be a bit confusing to those new to editing wads.