Half an inch in diameter (actually .501"). Standard length is 21/2". Hasn't been available in the US since the late 1930s, but apparently is still manufactured in Europe.
The value will vary depending upon the specific shell. Go to gunbroker.com or gunsamerica.com and look through their classifieds. You'll find old paper shotgun shells for sale by the box, and by the shell. Good luck.
Look here: http://www.dave-cushman.net/shot/shotshellloads.html and here: http://www.dillonhelp.com/sl900manual/shotshell_dmnsns.htm
Ensure you look at the stamped shell gauge on the side of the receiver (12 or 20 Gauge). Should be able to take 2 3/4" and 3" shells.
Check the receiver
Look up the Marlin shotgun with the same model number.
The Mossberg model 83B shotgun in 410 gauge was a shotgun that held either 3,or 4shots which had a internal top loading magazine.these were made from 1933-1946.It should be noted that this model was a bolt action shotgun with usually a 24in barrel.Other lengths could be ordered I believe.
a shell
16 gauge what? Wire... sheet metal... shotgun? No, seriously; look up a guage thickness chart for the material in question.
Look on bottom of receiver and barrel
Value of a shotgun is driven by several factors. Condition, model, area of the country it is sold. Look at one of the used gun sites like gunbroker.com. Then compare.
Whether it is a Savage/Stevens of any other modern shotgun, look on the top or side of the receiver and it will give you that information. For example: on a 12 gauge it will say 2and3/4 inch only or 2 and 3/4 inch and 3inch magnum. The gauge is equal to the number of corresponding lead (as in metal) balls that it takes to equal on pound (troy)
Browning.com has sn tables you can look it up on.