Most important: Check if the grid you plan to use has enough fields. The number of the fields of a grid can be found in the description of the grid, eg. 24 x 30.
Pen and Paper
Sometimes you do not even need a grid. And when you do use a grid (for measuring distances e.g.), it is usually a 12 mm grid, hex or quadratic, because the magnetic buttons are intended for 12 mm grids on a A3-Gamerboard.
Tabletop
Usually the game defines if you need hex or quadratic grids. And the size of your miniatures and their bases define the with of the fields of the grid. Mostly that is one inch.
Sometimes it makes sense to use a smaller grid (eg. 15 mm instead of 1 inch) because then you have more fields on the grid. The miniatures and their bases may be a little bit larger than the fields of the grid but when you usually use only few miniatures this does not really matter. Just try it with a print of a 15mm-grid.
Grid-Downloads.