This is a case of More Is Better! I'd check with the printing company for their recommendations. I got these .jpg specs from the website of my local photo place for fine art prints.
11x14 1650x2100 1.9MB
12x18 1800x2700 2.8MB*
16x20/24 2400x3600 4.9MB*
20x24/30 3000x4500 7.7MB*
*Larger files at 300ppi will produce better quality prints.
I have printed an illustration at 24x36 on a wide format hp printer. The file was 4900x3200 3.5MB and it turned out nice. The printer was mostly meant for CAD drawings so it's not "fine art" quality but good enough for hanging around my office.
Viewing distance makes a differance as well. Just like cars, it may look better at 10' than 2'.
Where are you gonna hang that bad boy?