They are a standard "size", it's the microns that vary.
You'll find a wide degree of differing opinion on what micron size to use, this is what really determines the quality of the filtering. I'm good about knowing where my water is coming from, so I generally use one that isn't as "tight" as other might use.
Keep in mind that this will affect water pressure, as the filter produces a pressure drop. A filter for a house on city water can have 100+ PSI pressure pushing the water through, though generally, hosehold pressurre is regulated to 50+/-PSI. The little pumps in an RV push nowhere near that kind of pressure.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter