Most VTTs used a standard resolution of 50 pixels per grid square, but 100 pixels seems to be gaining popularity of late.

Battlegrounds (aka BRPG) uses a resolution of 122 pixels per grid square (or hex), but the maximum map size is 4000 x 4000 pixels.

As RedRobes said, bigger is usually better, since users can easily downsize a graphic to the size they need, whereas if you try to enlarge a graphic beyond it's native size you will get pixelation/blockiness.