PKGranny, no my finished block was 12 inches. I used pieces of very old tissue patterns cut to 6 1/2 inches for the foundation of the string blocks. I laid and pinned the first one kitty corner and then added strips on both sides. I then trimmed to 6 1/2 inches. I shortened my stitch length and the tissue was quite fragile so tore away very easily. I then put 4 of those together for the 12 1/2 in string block.
For the alternate block I made eight 3 1/2 inch (unfinished) HST's by putting together four 4 inch coloured squares and four 4 inch background squares, sewn and trimmed. The corner squares were 3 1/2 inches unfinished and the center square was 6 1/2 inches unfinished. It was my design decision to not mix up the HST's. I felt the top was busy enough as it was. I did add borders in the same background colour because I could not find any other fabric that complimented the design. The final size was 90 inches square.
but there is no reason why you could not make 24 inch blocks. You'd only need half as many!!