What size hole do I drill for this bulkhead?

[Thricelll]

This is my first time doing my own drilling and I have no clue what size hole to drill the hole in my tank. Do I drill a one inch or 1 3/4 inch hole? The description at MD's website says 1" and 1 3/4th

http://www.marinedepot.com/md_viewItem.asp?idproduct=FT1155

 

[Southern Boy]

1 3/4"

 

[IMM3DOORSDOWN]

use the rubberseal on the bulkhead and trace it onto your tank.