I think I got the question; first 5x7 isn't amateurs size, it is the size for kiddies that have their cheap photo's processed in a high street shop. 12 x 18 inches is substantially harder.
Colin might say this is easy, but in fact some does abstract photo's like 24 minute water, which hasn't really got a lot of resolution.(can't have)
You can with a modern printer GET sharp in the sense if you look close about a foot away from a 24" print largest size, it looks sharp with the right paper and ink.
But a lot going on with Nyquist, the best possible ect ect.
It is more fun than just trying for a computer image, and definitely harder as well. Unless you just take ordinary stuff following rules, high contrast feely images are difficult.