Three things to consider. First is the thread size of the lenses you intend to use the filter on. It could be marked on the lens itself or on the inside of th lens scap, and could be something like 49mm, 52mm, 55mm or even bigger. A search on the net for the lens could also give you the information. You could also measure it yourself and compare it the list of the standard filter sizes. When you consider it to use it on more lenses: get the biggest size and use step up rings as Prest suggested.
Second thing to consider is how long you want to expose. For fast moving water you will need shutter speeds somewhere in the range of 1/4 sec to 1 sec to get those white washed water paterns. For moving waves on lakes and the sea 10 sec and up is necessary. In the first situation a 3 or 5 stop filter will do the job, for the second one you will need a 8 or 10 stop filter, depending on aperture, filmspeed and weather.
The third thing to keep in mind is that film loses speed when the shutter speed increases above 1 sec. This is called reciprocity failure and the effect differs from film to film. For HP5+ for instance, when you measure 2 sec, you should expose for 5 seconds. It gets worse with longer expores and you might find yourself doing exposures of minutes while measuring a time of 20 sec. Do research on the film of choice and make yourself a table because the effect is very noticable and there are huge differences between films.
