If you are intending for your battlemaps to be print-and-play (my favorite), you will want to go with one of the 2 primary grid-size conventions, either 1 inch square or 25mm square. As Gaemon says, Virtual Tabletop [VTT] uses either 100px or 50px as standard conventions (50px is more common, but 100px allows far greater detail), but then again if you are going down that route PDF probably isn't the best distribution format.

If your maps are intended for printing keep in mind that lighter tone with greater contrast is better than dark and subtle. Most home printers can't handle exceptionally high levels of detail, no matter how much the manufactures tout their products' capabilities. It is also a courtesy to the customer, saving on ink is always appreciated.

I personally would aim for an image size per sheet of no more than 8 in X 10 in, or 200 mm x 250 mm. This would ensure it could be printed on both North American Letter or A4 paper sizes.

That's all I can think of for now.