Cherbyl, you are asking about Block 11. The small half square triangle units are 1.5" FINISHED so they are 2" Unfinished. They would be made from squares cut 2 3/8" if you were doing the squares method and wanted to sew exactly perfectly. If you check out my Month 4 Blog you'll see I used a grid method instead, drawn 2.5".
You are right that the little numbers shown in the diagram say, incorrectly, 2 7/8" squares cut in half. I will bring this to the attention of those who can fix it. Thanks.