as mentioned bigger is better due to quality loss when you scale up. In addition to your hardware being able to handle big maps another thing worth considering is where the map is "going". Are you going to use it on a screen in a virtual gaming table program - then what you see at 100% would be the detail level you're going for. However, if you want to print it later on, a resolution of 150-200 pixels per inch would be prudent (300 if you want to print professionally) - so multiply the inches wanted (like for instance for letter format) with 200 and you get the optimal size for that document. The biggest map I'm working on at the moment is 16,000 x 12,000 pixels - so at professional printing quality that translates to 53 x 40 inches (134 x 100 cm) or a bit bigger than your desk top printer can manage