Seems like a good project you have there! For most people the size of the sump is determined by the size of their cabinet and how much thy can stuff in there
the bigger the better as it will increase your water volume and thus make your system more stable.
If you have absolutely no know-how I would highly recommend reading A LOT more before I leap into this! Setting up a reef tank that size will usually result in thousands of dollars investment and there are a lot of rookie mistakes you can avoid if you know about them!
A couple of things to start with:
Drilled tank vs overflow
Metal halide vs t5 vs led
Sump vs refugium
Coral types and live stock in general
How will you do water changes?(carrying 75 gallons worth of buckets every week isn't as much fun as it sounds)
Good luck and feel free to ask more!