For Pro Products, you have to call Bob and give him the specs like ambient room temp, type of snake, cage dimensions, upstairs or downstairs, finished basement etc. He then can calculate the size needed based on that info. Its done so you get one that will get you the desires heat yet small enough to not take up 3/4 the cage. An example is for my ProLine cage, I ended up getting a PH3 which is their 65 watt one. Works fine. If I remember right, it was around $110. Its about 22x9x1 just looking at it in the cage. Either RBI or Pro Products will work. RBI uses an ABS case and wood screws to attach it and are cheaper. Pro Products use white aluminum housings and use bolt and nut which means you gotta drill holes in the cage to mount it. From what I gathered, it seems Pro Products are about $10-20 more expensive than RBI. Again, either will work. I'll probably do RBI ones in my AP cages so I can stack them easier since I wont have to try and mess with bolt heads on the top of the cage.