Terrain & Furniture
Walls and floors are 32x32, other things may vary
// TODO: add terrain example image
Again, you should size things accordingly to their size in the world.
Keep color value level in medium range
// TODO: add image with color levels explanation
It's important to keep the value level of background objects (terrain, furniture) at about 60-80, because it helps define foreground entities (e.g. monsters) better.
Keep table surfaces consistent
When drawing something similar to a table keep in mind that currently table surfaces usually occupy the top 19 pixels of a tile, and items may be optimized to look placed in that area.