Not done much pcb design, but the general rules I was taught to follow are basically:
1. Think about cooling, if there's anything that's going to get hot position it sensibly. Remember that the temperature /current rating for components may be based on having forced air cooling - check the data sheet! You can sometimes use the pcb as a kind of heating for ICs etc. Have a read of
http://www.electronicproducts.com/Thermal_Management/Heat_Sinks_and_Thermal_Materials/PCB-cooling_techniques_and_strategies_for_IC_packages.aspx
2. Breakout every pin you can, you'll thank yourself later. Add test points and plenty of ground pins just in case.
3. Just because you /can/ make it tiny doesn't mean you should - may be tempting to get tiny boards because of the price, but make sure they're useable /solderable/mountable etc.
Seed studio always gets good reviews from the guys I know who do this stuff, probably not quite as cheap as your link but quality seems to be fairly high and consistent.
EDITED: 13 Feb 2016 13:03 by KOSWIX