Not a stupid question at all, Boss.
If you are looking for an AR, you will be seeing mostly 16"ers or maybe 16.1"ers. This is the standard carbine configuration, and you won't need a tape measure, as all dealers know the difference.
The other common AR is the 20"er, which is the standard rifle configuration (and is the measure of the venerable M16)
so you will likely be seeing carbines in 16, and rifles in 20
If I could make one recommendation, to would be to highly consider a flat top configuration (A3) since it opens up future options to you that the A2 (permanent carry handle) does not.
welcome to the site, and welcome to AR15's