Most revolvers will have similar lines, but they are scaled differently, so it is difficult to appreciate their real size unless you're seeing them in real life.
The frame sizes you listed are designations used by Smith & Wesson. Their scale is somewhat similar to other manufacturers.
I-frame -- very small, .32 caliber frame. This hasn't been used much for a very long time.
J-frame -- small frame. This is their typical snub-nose revolver frame. It is pocket sized
K-frame -- medium frame. This is the mythical belt gun of Smith & Wesson; the legendary military and police, the combat magnum, etc.
L-frame -- beefed-up medium frame. This frame was created for the 686 and 586 revolvers. The Combat Magnum (K-frame .357 magnum) was known to take ouchies from firing hot magnum loads too often because the 6' oclock on the barrel was thinner than normal. The cylinder and frame were slightly enlarged to make a much stronger revolver. Since then some other calibers have moved to the L-frame, such as the .44 special, but the .357 magnum still dominates this frame size. Great guns.
N-frame -- large frame. In the old days this was the .44 caliber frame. It is also the frame where the magnum cartridges originated, the old revolvers simply named ".357 magnum" and ".44 magnum." These are not small guns. They are, however, known for being some of the finest and easiest to shoot.
X-frame -- Extra large frame size. The .500 magnum and .460 magnum. Carbine size. Very, very powerful. Very fun.
I hope that helps.
Edited to add: No single engineering or significant mechanical differences are intrinsic to each frame size. There are small differences, but nothing too obvious. In this regard, Smith & Wesson revolvers have changed very little in the last 100 years. There are double action only guns, but other than through custom work and Performance Center special editions, this is somewhat unique to certain J-frames, particularly the models 640, 642, and 442.
