Try the Blue Book of Gun values
It will usually be marked somewhere on the barrel.
One can make a pistol much larger than it needs to be, maybe even having an extra-long barrel would make a pistol larger, technically. I'll assume you mean the barrel diameter. Although it would seem that a .380 and .38 would have the same barrel diameter, namely, 38/100th of an inch, that is not usually very accurate. The .380 ACP has an actual barrel diameter of about .35 caliber, or 35/100th of an inch. The .38 Special has a barrel diameter of .357 caliber, or 35.7/100th of an inch. So the .38 Special has an ever so slightly larger diameter.
It should be marked on the barrel.
Depends on the caliber, barrel length, projectile weight.
What is the caliber, barrel length, finish, grip material, markings on barrel?
The M9 is a standard 9mm round. the M9 is actually a .355 caliber
Caliber is the size of the barrel's inside diameter, in fractions of an inch. A .25 caliber pistol fires a bullet that is about 0.25 inches across. Pistol can range from very small (about 10 caliber) up to very large (75 caliber) and there are hundreds of different cartridge sizes in between.
4 inches including chamber.
ABOUT $150, depending on condition.
Need to know caliber, barrel length and whether it is a revolver or semi-auto pistol in order to answer.
Can't be answered with any degree of accuracy without a detailed description of all markings, features, condition, finish, box, caliber, barrel length, etc..
Made in 1951