@_arianao If you've made alphas before, just count the number of rows per inch according to your knotting style and divide that by the number of rows in the pattern. I usually knot 14 rows per inch, so I know this pattern which is 85 rows (85 divided by 14 = 6.07) would be about 6 inches for me.
Depending on how long you want your finished piece to be, you can either add or subtract the same amount of rows on each end to keep the design centered.
I also take into account the length of triangle ends if I'm using them and add that as well. 😊