Length: Determine length of top, 4 ft., 6 ft. or 8 ft. (from front bow to rear bow).
6' Tops are the most popular length. These will extend 3 feet forward and 3' behind the point where the main hinge is mounted.
8' Tops offer a larger shade are and are popular for larger boats or instances where you want more shade. They will typically cover 4 feet forward and 4' behind the hinge mount point.
4' tops are suitable for smaller boats, or applications like fishing where optimal clear railing space is desired.
If you are measuring to match an existing bimini top, fully extend the top and measure from the front bow to the rear bow. Don't measure the cover as some will have extra flaps etc.