Here is a section of a throw quilt I made a few years ago that is now displayed on a wall. The back is a single width of 45″ fabric that is about 54″ long. The diamonds were cut from 3″ wide strips cut at a 60 degree angle. The sides of each diamond are of equal length and measure about 3 1/2″. The seam allowance is 1/4″ from the cut edge. The finished longer diagonal of each piece measures about 5″. Counting across the second row of blocks there are 7 blocks. At 5″ length per piece that equals 35″. Without the 5″ border on either side, two more pieces could fit across making the total width 9 x 5″ = 45″.
A larger size diamond allows more of the fabric design to show which is nice for novelty prints. The quilt below uses a smaller size diamond. It has 28 diamonds across the width. Total measurement is 86″ wide. The diagonal of the diamond piece would be 86 divided by 28 or 3.07 inches. In order to appreciate the overall design in a quilt, I think 12 diamonds across might be the minimum. If I used the larger size diamonds as in the wall quilt above, the width without border would be 5″ x 12 or 60″ wide which would be about the size for a double bed. The length measurement will be longer due to the setting of the larger hexagon units.