The jelly roll quilts made with 2.5" strips will always be 32" or 64" long, unless you make some adjustments. To make one 96" long, you would need to add a strip after sewing the first 2 ends together. The added strip would need to be 1/2 the length of the first strip.
So, for example, if you used 80 strips, sewed the ends together, then added a piece of 40 strips. (or just used 3 pieces of 40 strips, with the ends adjusted so that they won't all end up the same length at the beginning of the strip)
80 strips x 40" = 3200"
add 40 strips x 40" = 1600"
first sew 1600" x 3 x 2" = 6"
800 x 12"
400 x 24"
200 x 48"
100 x 96"
It would come out a little less than 100" from sewing the strips together, and even more less than 100" if you used diagonal seams. With butt ended seams, you'll lose 1/2" per seam. With diagonal seams, you'll lose 2 or 2.5" per seam.
I like the diagonally attached fabric, and also like the squares of an accent fabric attached to butt ends of fabric. I've usually seen those as black squares, but any accent color would work as long as it is contrasted from the rest of the strips.
So, to get lengths in between those, you'd just add another strip - when you have 4 strips sewn, add a 5th strip the length of the 4 strips. It's not difficult to calculate. And you can change the width of the strips to change the length of the quilt.