Tile sizes can be confusing and for some reason they actually make a difference in render speed! But why bother to figure it out when you can have a simple addon do it all for you?!
This addon takes care of that. Just enable it and you should never need to worry about tile sizes again!
It automatically calculates the best size when you change the resolution, or when you change from CPU to GPU and back, or accidentally change the tile size yourself! (although if you do want to set it manually, simply toggle the Auto Tile Size button)
(Currently for Cycles only, though I’m investigating what determines good tile sizes for BI)