I got a really nice Carver bimini, made with Sunbrella, for my boat. You really just need to figure out how wide it should be, how long the supports should be (i.e., how much headroom is under it) and how long much of the boat (from fore to aft) you want covered. The length (fore to aft) will generally determine how many 'bows' (metal u-shaped bars) are needed to support it.
For mine, I determined the height by standing and measuring from the gunwales to the desired height of the bimini (giving me a couple inches of space above my head when standing). That worked pretty well.
The width is generally measured from gunwale to gunwale, across the the boat side to side. I went up to the next size so my bimini is just a bit wider than the actual boat; that provides a bit more shade when the sun is not directly overhead.
Length can be whatever you want. I measured mine to be long enough to cover the driver's seats and the rear bench seat, leaving the sunpad (in back) and the bow seating (in front), open for sunbathers. That lead me to a 3-bow bimini.
Most web sites that sell them (including iboats, I believe) have some instructions for measuring. You have some fudge factor to play with, so they are note super-critical measurements.
