The tiles for my top down 2D shooter are 32x32 and the player and enemies are too. But obstacles like walls and other things can of course be smaller than this - the smallest being 8x8. This causes a problem where a single 8x8 barrier intruding in a 32x32 tile will cause that whole tile to be impassable if my AI Grid is based on 32x32 tiles, like as follows:
This results in a grid with more impassable area than it should have. (On the left the red being the AI grid's impassable area, and on the right, the green being the box2d barriers for the same map section).
The other option is to make the grid 8x8 based and then the grid will perfectly calculate the actual impassable areas but it will create paths too narrow for the 32x32 enemy to pass through. With design I can make the first way (pathfinding grid of 32x32 tiles) work but it's not ideal since you're limited to placing everything in perfect tiles so as to not create impassable areas that are in fact passable.