How much bigger do you want to make it? When I enlarge a quilt like this, I add more rows to top and side to make it what I want it. The solid diamonds are 4" finished which on point finish 5.75", so take size you want and divide by 5.75 and you will know how many rows up and across you will need to make that size. It probably won't work out evenly inches wise but if it works out a little short, then add another row. On the diagram there are 7 rows across, so the middle, longest row diagonally uses 7 solids. Say you need 15 rows across, you will need 15 diagonally, whether the center row works out to be four patch or solid.
You can do it. If you need a visual, print out a grid and mark off as you sew. Have fun!!