if you are screen printing your design, check with your screenprinter to see what their max imprint size is. i work within a 13" x 17" area which is pretty large. most of my designs end up being around 10-12" across and start around 2.5-3" down from the collar hem.
it's hard to say what is "typical". it really depends on the design and the size of shirt you're printing on. best advise is to get yourself a shirt, print your design at different sizes and place it in the location that looks the best.
test the waters before you commit.
whats the typical size? 
