If you're planning to have your image cover the entire canvas (side wrap included), the recommended resolution for each size is listed below:
15x10:2250 pixels wide by 1500 pixels tall
24x16:3600 pixels wide by 2400 pixels tall
30x20:4500 pixels wide by 3000 pixels tall
36x24:5400 pixels wide by 3600 pixels tall
The editor will give you a warning if your resolution is below 300 DPI.