It says 5/64 After the edit.
Dave. The hole goes inside the sealing gasket perimeter. Just drill thru the cap where it will let air pass out of tank but not damage any valves, seals or other cap feature.
It is probably best to have rubber hose between tank/body and metal line that is hard mounted to frame. There is some movement between body and frame. Likewise at other end up to pump. There is movement between motor and frame.
The hole could be small, as Carl says, as long as it doesn't whistle like a tea kettle
Glad Carl confirmed that the top draw works. Other AD's will be interested in that. I was worried that since the 5 hole is not symmetrical and is "clocked", that that might cause the outlet to be aimed wrong and/or the pickup/float to hit side of tank.
