For a bimini, you have a lot of flexibility on sizing... most ask you to choose a width, the length and the height (usually measured from the top of the gunwales).
For the width, just measure the distance from the port gunwale to the starboard gunwale. I think I added a couple of inches so the bimini is just slightly wider than the area of the boat beneath it.
The length is determined by how much of boat you want shaded. I wanted the cockpit and rear seating area shaded, so I just measured how much bimini would be needed to cover that area.
For height, you first need to figure out the distance between the deck and the bimini . Since most bimini makers want to know the distance between the top of the gunwales and bimini, you figure out your desired overall height, subtract the distance from the deck to the top of the gunwales, and the remainder is your bimini height measurement. For example, I'm 6'-3" and wanted to be able to stand while driving without my head hitting the bimini, so I made the overall height 6'5". If the distanced from deck to top of the gunwales is 3', that would make the height of the bimini itself 3'5".
Hope that helps!
