Measure the length from side, top to the other side and add 1 -2 inches for a seam then do the same for the width and multiple by 2 to account for the underside and add 1 inch for a seam. From that you can determine the yardage required based on the selected fabric width, typically 54 inches. See the calculator below. As far as fabric, that is a personal choice.............you probably want something that is durable, moisture and fade resistant and easy to clean. Some fabric choices for interior cushions include Sunbrella, Nauga Soft with a leather look, Ultraleather and leather. Refer to the article on fabric selection below. Also as far as foam, firmness is anther personal choice..........measure foam thickness and add 1/2 inch on each side for compression and select the firmness you want (we like extra firm). See the foam sizing form one supplier below. Just did that for our settee cushions. If you plan to DIY there youtube videos out there (check out the one from Sailrite). Another option is to take your cushions to marine cushion fabricator and have them source the material and make them for you.
Interesting article on fabric selection:
A Fresh Look
Fabric yardage calculator:
Fabric Calculator by Sailrite
Foam sizing:
Welcome to Buyfoam
How to make cushions by Sailrite: