The general rule is 1:1 or 1:1.5 size with memory to the page file. To be more specific check what your committed memory is now, if it's full you should increase the page file if it's too high shrink your page file a bit. Then check for page faults using performance manager and compare your page faults per second, disk io length, and other disk metrics to see if you have an excessive amount of paging going on, if so you probably need more physical memory or to optimize what memory is being used.
