Thinking man. I'd say your average full front or full back design would be around 11" wide and maybe 10" high. Don't think that that means you should fill every square inch though. Sometimes more is less. Still, I fall victim to trying to fill the whole thing up.
In photoshop, you can work at 200 dpi and still get good quality on a garment print which will help to keep your file sizes down. Working in RGB mode will also help reduce it. You may already be in RGB tho. Photoshops OK if you're working
DTG or simulated process. However, if you're working in Illustrator (or other vector application) you'll have much smaller files to work with, spot color capability and you'll be resolution independent.