This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.