This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You can specify a fixed size, and the buttons would all be that size. But you do that by specifying the size of the whole component. If that's still confusing, have a look at the API documentation for GridLayout (<== follow that link). It has pictures.