The standard grid is 6" boxes.
But the great thing about a gridded game, is that you can make the boxes the size to suit your units. In Simon's photo's above I think it will be using a 20 or 30cm grid, so that the units are big and impressive. If you have smaller units (particularly with smaller scale figures) you can drop the grid size and either play on a smaller table, or get a bigger battlefield on your table.
Basically the grid needs to be a little larger than your unit frontage, we play with 12cm unit frontages on a 15cm grid. With units having about 5cm depth, this allows 2 units deep in a box easily.
This is a FKaP game, using 10mm figures with 12cm frontage, the mat is a pre-printed Deep Cut one, where the grid is barely visible
A different game, using an old Citadel mat, with the corners marked in sharpie. These are quite visible, but I could (should) have done these with a finer pen, and probably smaller