If one is already in the tank- it's best to remove it and get the #'s of of it.Some tanks have baffles in them and use a sender that looks like a tube.
Other tanks use a sender with an arm and a float.
Be sure to clean the area under the gasket well and to use a new gasket.The E-10 fuel causes the older gaskets to leak sometimes.
If you do not have a sender then remove the plate on the tank and measure the depth of the tank.From there most Boat supply stores can order you a sender.It does not matter how many gallons the tank is- just the depth.Use a flashlight to look inside for baffles.The tube type senders are more expensive but give a better reading IMHO.