I think mine are good for up to 50 gallons and I use them in smaller tanks (25-30 gallon?) You probably just don't want to go bigger than the recommended size or it can't keep up.
I'm not sure which ones you are looking at, but some of the plastic tank appropriate heaters do need to have a metal cage type thing put around it. Make sure you get whats made for your type of tank.
I am currently keeping my aged blind appy in his own stall with a run/corral and the dang water freezes of course. While checking on the links in this thread I noticed this product. It looks like something that might work for me. Has anyone had any experience with this type of insulator? I don't have access to electric plug near enough to use a heated bucket. Plus my QH gelding would probably find and eat the dang wire.