Welcome to the forum.
Go to Tools>options>document properties>units. This is where you can change it globally.
To add onto his point you will have to use the "custom" bullet and change the pull down to ft or ft & in.
All this does is just change in the default so you dont need to type in the units each time. You can always type in any unit it doesnt matter what your global setting is at. When you place your dim on the sketch (using smart dim) just type in the unit you want (in, ft, cm, mm.....) after your number. you can even mix the units ex 2in 3cm
In your case if you want ft and in you can type in both. ex: in the smart dim box type in 12' 6" or 12ft 6in will both get you 150"
However the sizes you are asking for are out of the size sw can handle
SolidWorks is limited to a part size of a 1 kilometer cube centered on the origin.
I think you are requesting a part size larger then SolidWorks can model.