The best way is Frances' method. If you are designing a 2" x 3" card, make the page size 2" x 3". Then, when you export your image simply chose the desired resolution for your output: 96dpi for web work, 300 dpi for images sent to printers, 600 dpi for high-resolution print work, or whatever other resolution you desire. YOU CAN type in a resolution in the export box, rather than using one of the defaults if you want something that isn't listed (such as 600 dpi).

Best wishes,
David