Realistic spaceships have large engines and huge fuel tanks because they need to carry (and accelerate) loads of reaction mass. There are plenty of design studies and examples out there, but the basic point is you'll end up with a huge percentage of fuel to propel even a tiny payload...

If you are going for space opera, then ships are any size you want. Most SF authors probably just make stuff up as they go along, some may have rough guidelines and I am guessing very few actually have set "rules" on ship design.

I've looked at ship sizes myself some time ago, mostly trying to figure out what a "tramp freighter" would be like, and I've based my comparison on Traveller, H. Beam Piper, and some real world stuff:

http://enderra.com/2011/10/14/starsship-sizes/