They are listed as the diameters of the fitting already, 1/2" and 3/4".
You are able to use this Y fitting with the Red Sea tanks, but you will need to adapt the Red Sea return to loc line using the 25mm to loc line adapter. Have a great day!
Not into a slip bulkhead. You would need to use a bulkhead with threads on the inside and a NPT x Locline fitting. That fitting screws into the threads on one side and then has loc-line fittings on the other side.