Terriamn, love the desert. Your pix are beautiful
Lovely new blocks, MonaG.
Bonus blocks. As bonus blocks are a gift, it seems unreasonable to say you "should" make them any certain size if you wish to make some. However, Kassaundra is right that if your goal is to see them used in a quilt, presumably the one you've made your required block for, 12 1/2 inches would likely be the most appreciated size. But again, these are not a required part of any Boom swap.
And we ARE stressing that Boom blocks be 12 1/2 inches square so measuring as you go to ensure their size really is important. Everyone should double check them before packaging and adjust as necessary. Thanks!