Mr.T has it right.
Going ot add a little more. The frame size is a letter designation only. The makers figured the frames needed naming to differentiate the types of frames and that was the easiest way was to letter them for namesakes.
S&W is and Colt started doing that many decades ago.
J frames and their kin are the 5 shot snubbie class that S&W started as far as naming went.
When S&W came up with their 6 shooters for .357 magnum and their kin the K frame moniker was used.
K framed .357 magnums were taking a major beating and malfunctioned after the much higher pressured 125gr. JHPs were introduced that are favored for police and civilians alike. So S&W created a new frame slightly larger or about the same size, but able to take the pounding and extreme pressures better than the K frames. Less flame cutting or torch cutting of the forcing cone was noted also.
L frames are heavier, but take the steady diet of .357 magnums of any sort much better than the K frames.
Taurus offers K frame like revolvers with the models 65/66 or the 82.
J frame like offerings are the models 85/650/651/850/851 series. Rossi has the 361/362 series.
Tracker and Raging Bulls are not L frames, but are designed to take similar poundings from high pressure ammo.
X frames as Mr.TuffPaws has said are for the highest pressures and are the largest frame size that S&W puts out.
N frames are the large .357 magnum, 44 Special/.45 Colt frames for S&W.
Colt's D frame was their Colt Detective Special and it's kin. These are 6 shot snubbies. Rossi's 461/462 are similar size, but have different means of internal operation.
Might have missed a few designations of examples as to other Colt and S&W designations, but that about covers most of it.
