Well, I tend to set units of measurement when starting a project by going to windows -->settings/preferences --> preferences -->settings. Each grid square will represent 1 unit. So, if you set it to inches, each grid square will be in an inch. Then, you can just count grid squares or figure out the model size beforehand.
I'm not quite sure how this works along with grid options (display --> grid option box). Maybe someone else can shed more light on that.
I've found this helpful when building things to scale...like making wood planks that I want as 2 x 4s and what not.
"Terminat Bora Diem, Terminal Auctor opus."