Your discharge hose size will more than likely be 3/4". That is the size from 500GPH to around 1100 GPH, which would be the typical size up to the mid 30' range.
Look at your bilge pump now and determine if there are 1 or 2 positive leads to the pump. If there are two, then you would connect 1 to the helm switch and the 'hot' lead to the switch. If there is only one positive, then the float switch goes in between, however this requires the dash switch to always be ON for it to work.
If you buy a new pump with an internal switch, make sure it is not the kind that has to power on to sense water. Personally, I prefer a passive switch that doesn't draw power unless actuated.