ShadeMart8' x 14' Beige Rectangle Sun Shade Sail Canopy SMTAPR0814, Water Permeable & UV Block, Heavy Duty, Easy Install & Durable for Patio Garden Backyard - We Make Custom Sizes

Q: Will this size work for 8'x12' structure with 4x4 posts?

It's tough to get the needed tension unless you size down. For an 8 x 12 structure, I would recommend a 6 x 10 shade.

Q: My driveway is 11' wide... Should i get an 11' or 10' shade to cover my car?

You need to decide the shade area you want then find the mounting point for the shade sail like roof, poles, wall. Leave 0.5 to 1 ft space for the hardware which you need to use when you take the measurment of the mounting points. After all the steps above the width of shade sail should work for you.