Yah... estimating is really hard to do accurately. This is why I bought a plastic ruler that I stick against (or in) my tank to measure my fish. That way, when I am trying to communicate info about my fish I can just go measure it to see how big it really is. For example, I have two bichirs and they seem pretty big to me as I've had them since they were really small, but they are only 5 inches (tail included, even)! They will probably double in size or more.
Obviously, this doesn't shed much light on maximum fish sizes. In fact, in official descriptions of fish, the size listed is not a "max size" but simply the size of the specimens examined. My preference is to find a site that is generally trustworthy (I almost always refer to planetcatfish.com for catfish) and base my information off that. Even with reasonably trustworthy references, they can have errors or just be mistaken, so it's really important to take every bit of info with a grain of salt.
In cases where the "max" size of a fish is listed as a wide range of measurements, I ask myself "Since the biggest size listed for this fish is 15 inches, assuming it really can get that big, can I keep it happy in the tank I want it for?" If the answer is no, I think really carefully on if it is a good idea to get the fish or what I would do if it outgrew my ability to care for it. Doing something like this will help prevent the heartache of buying a beautify red-tailed-catfish, only to discover that yes, they really will grow to several feet long in the blink of an eye.