Some people measure fish in inches or pounds depending on the species. I've heard people refer to bass in pounds and trout in inches while others stick with pounds for all fish. I'm always interested to hear opinions on this subject. And as tnflyfisher pointed out, it is essential to get both length and girth measurements to accurately estimate a fishes weight.
Good example for this is that most fish in the 28-30" range in east Tennessee will probably weigh 7-10 pounds and perhaps up to 15. However, a fish out here in Colorado was recently caught on the Frying Pan
that was around 29" and had a girth of 24"+. The estimated weight was between 21-23 pounds!
I've caught some really thin trout over the years and also some that were pretty healthy with a lot of weight. Time of year makes a big difference also. Trout will eat heavily just before the spawn in preparation for the hard work ahead. After the spawn is over they are normally significantly skinnier and eat a lot again to make up for the time away from eating...
Personally I almost always refer to trout in inches, probably because I don't normally take the time to measure a trout's girth to get an accurate estimate on weight.
Good topic and I'm interested in seeing others' opinions.