Change OSX system font [duplicate]

Apple doesn't have an option for you to do that. However, you can try TinkerTool. You can download it at no cost.

Screenshot taken from TinkerTool website.

Unfortunately, there is no system-wide way to make the content fonts larger across all applications without changing the resolution.

It is possible to change the fonts used by the system, as the other answer points out so well. You may find Tinker Tool to be enough of a solution for your problem.

This question attempts to step through how to make individual programs increase their font size, but it focuses on Lion, not Snow Leopard. Still, you may find some helpful tips there.