Read the description of the contest carefully to find out what format the client requires.
For your submissions to the contest, we usually use the JPEG (.jpg) or PNG (.png) format. The final winning design should generally be provided as a vector file.
The most appropriate size for the competition is 800x800px. The only exception to this rule is Web designs, which are generally larger than, for example, a logo. In this case, it is best to keep a resolution of 1024x768.