The only stupid questions are those not asked

especially true in this hobby! I don't know the exact size recommendations other than the bigger, the better (at least in my opinion). As far as minimums... you want to ensure its capable of handling around 10x turnover of tank volume per hour with enough space that it will not overflow when your return is switched off. I am using a red sea reefer 350 sump on my 75 gallon tank (I think it's about 25 gallons) and it does the job I need it to. As far as drilling your tank, I would highly recommend it. If you don't feel comfortable doing it see if a LFS will. I wouldn't want to deal with a hang on overflow personally...
Oh, and welcome! It's great to have you here. This is a great place to be