I have 18" and 24" and honestly should have purchased the 36" instead. The longer they are the harder they are to maneuver. The 18" will get ya about 25" of rip (rail is around 7" wide) but the tape measure is only good to 18" so that sorta stinks. The 24" works well and the tape covers you to 24". However, if you get the 36" you can then use it on a shorter rail to crosscut your ripped goods for cabinet assembly which is typically 34.5".
I want to add that I have the 108" rail I tried using with the Rip dog guides and had mixed results. I ripped about 4 sheets of 3/4 birch for a small built in I was doing last week and wasn't exactly thrilled with the results. I spent quiet a bit of time calibrating the stops to make sure the tapes were aligned in hopes of being able to use this as an alternative to running plywood through the table saw. However, some of my rips varied by as much as a 1/16". I'm sure it's user error but I was very deliberate in keeping the stops snugged up to the plywood edge. You can hide this is some cases but when you have grooves and rabbets for your cabinet backs this just doesn't work well and is unacceptable. I ended up ripping all the plywood again on my table saw and will continue to use the table saw for all ripping and the parallel guides for crosscutting.