MY RHP's are 12 x 9 's I went with the Pro Products ones and you can't just order them, you have to give details about your cage construction/materials, room temps, occupant desired temps etc and they work out the appropriate size and wattage RHP for your application.
Yeah, thay are pricey but they do work really well and come with a long guaranty. For me the 16 RHP's were less than the value of one litter of BRB's, so was easy to justify. 




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Thanks,
Dave Colling

www.rainbows-r-us-reptiles.com
0.1 Wife (WC)
0.2 kids (CBB, a big part of our selective breeding program)
LOL, to many snakes to list, last count:
19.29 BRB
14.19 BCI
And those are only the breeders 
lots.lots.lots feeder mice and rats 