concrete base is 6" high and is 8 Ft wide. I am in the process of framing a 16 ft X 18 ft garage
I assume you are wondering what size header to use for the framing and how to support that header. You should: consult your local building department they can tell you. The old rule of thumb is 1" height of header for every foot of width. With an 8 foot span you should use double studs (jack studs) on each side to support it. So - for a 8' span with 2x4 framing a 4x8 header is minimum but my experience is a 4x10 with a nominal dimension of 3.5 x 9.24 is the smallest size header I would use. These days, contractors use 4x12s for most headers up to 12 in width.
Typically frame the opening to the size of the door, that is 16-0 18-0 door requires an 16-0 18-0 RO. But you can also check your manufacturers installation instructions or talk with your garage door sub or supplier to verify.