When I started with video standard American broadcast standard or dvd was 720 x 480 (540 for square pixels)  . With higher definition tvs you can go larger or just use whatever you like if its for use only on a PC. again www.videohelp.com will have the standards.  

 

The bigger your images the longer a render will take , multiplied by the number of frames of course. I try to keep things small because my computer is single processor and low memory.

some standards are:

1080p (1920×1080 progressive scan)

720p  (1280 x 720)

480p (720 x 480)

 

if you are making dvds you need to check what your software will author.