You will need 24 2.5 inch squares in purple. Combine 16 of them with background squares to make 8 four patches.
4 of these four patches will be used to surround each purple block as shown in the picture. Combine the remaining purple squares to make 4 two patches. They will make up half of a four patch in the center of the alternate squares.
If you have already finished your light blue squares you can complete two squares that are half purple and half light blue.