I drilled 1 3/4" holes to use a 1" bulkhead. I got my drill bit from my LFS
he carries a varity of sizes. It is one of those bits that you can't find at lowes or home depot. You will need to go to a glass shop or get one online. Check on ebay they usually have some available. When you get your bulkheads there should be a sticker on it to tell you what size hole you will need to drill.
When drilling your tank make sure to take your time and keep the area and the bit wet at all times. I had someone help me and they dripped water on the area while I was drilling. Also make sure to have a towel doubled up inside the tank where the piece of glass that you are removing will fall to keep it from busting the other side of the tank out.