I still use graph paper, a calculator (one can get a decent Texas Instruments one for around $10-$15) and pencil and paper. An eraser and/or white out is also good to have on hand.
A full size mattress top is 54 x 75 inches. Then add how many inches you want it to hang down the sides. Then think about sashings and borders.
I think I would start with 24 blocks - 4 x 6 block layout - that would cover an area of 48 x 72 inches.
Then you could think about whether or not you wanted to add sashings (strips of fabric between the blocks) and that would add some inches.
5 x 6 would cover 60 x 72 inches
5 x 7 would cover 60 x 84 inches
Sometimes what one does is determined by how much fabric one has - and then one improvises if one has to -
One can also draw three or four of the blocks on graph paper, and then copy that and cut and paste.
It is a good thing to see how the secondary pattern will emerge before getting overly committed to something.