1.75" for 1" bulkheads, use 1.5" pipe on the 1" bulkheads for the drain(s), and run the return up over the back as a single line, what size will depend on the pump and needs of the system. Running "multiple" returns for a tank this size is a waste of time.
Look here for what type of drain system to use, considering it is a blank slate tank, you should do it right the first time:
http://www.beananimal.com/projects/silent-and-fail-safe-aquarium-overflow-system.aspx
Where to place the holes is the current most frequently asked question in the monster thread:
http://reefcentral.com/forums/showthread.php?t=1541946 in the latter parts of the thread.
PVC pipe sizing is very simple. PVC pipe is identified by the inside diameter. so 1" refers to the inside diameter. However, this is a nominal (in name only) size, as the actual inside diameter varies, with wall thickness. That said, 1" PVC pipe will fit any 1" fitting, or device that is also PVC, so there can be no confusion.
Adapters can be a bit confusing at first, such as what the heck is a SPIGOT street ell? Such confusion does not need last long, as typing "SPIGOT street ell" into Google will result in many pictures, as well as descriptions of the part and the meaning of "SPIGOT." Other terms you will run across are: "MIPT" which is Male Iron Pipe Thread, "FIPT" which is Female Iron Pipe Thread," and "Socket" or "Slip," which connect with a solvent cement.
Bean's article (linked to) has an image showing the parts needed to complete the drain system.
