Grid Size Based on Distance from a Point

Ok. Maybe I'm just rusty or maybe this is actually a complicated problem. First. I'm trying to put a square grid over an irregular 2D surface. The size of the grid should be based on the distance from a point(s). Distance 0-15, Grid size is 2x2, distance 16-40, grid size is 4x4, beyond that, grid size is 8x8. I'd appreciate any help figuring this out. Thanks!