it's not that hard a question really.... is it?
when setting up a res.. you need to do a dry run without plants and make sure you don't have water all over the place cause your plants aren't draining to the res fast enough..and you don't want your res to run dry when you are feeding either. you will make a mess.. but once you have it running and not flooding the floor, or the pump running dry.. shut your pump off, mark your res and then dump your res somehow so you can measure what's in it. (once your pumps stop running and all run off drains to your res.. you will know where it needs to be, just dump your res into a marked 5 gallon pail, I use a wine kit bucket that has gallons marked on it to mix nutes).
any questions, PM and I'll help you out. It's not a hard question to answer.. but no one can tell you how much your res holds, so bear with people telling you they can't tell you, cause... they can't.