You need to do 2 measurements:
-Interior diameter of the downstem. There are 2 standard sizes of replacement sliders, not including glass-on-glass. You need to know this number.
-Maximum length that your new slider can be without making contact with the side of the bong, etc. This is approximately the length of the downstem, but you shoudl check. Measure by sticking a chopstick into the bong as far as it will go before making contact, then draw it back about an inch and pinch it right where it enters the hole. That's the longest possible slider your bong will safely accept.