Well, that's a rather personal question!
Actually, it's not the size of your grid, it's the spacing (okay, now even I have no idea what I'm on about).
For my True20 games we usually fly without any grid, and just use our imaginations, sketching on the map (graph paper) when that doesn't work.
For D&D I have three filing cabinets full of various Dwarven Forge pieces that I use for a lot of stuff. Sometimes doesn't work for purchased adventures, but when I make my own maps they work. I can cover our 4x8 gaming table with pieces almost entirely. But no grid.