I would suggest against doing this. The button sizes and shapes in a UIActionSheet are designed the way they are for accessibility reasons. They are also consistent across all applications on the platform, so users are familiar with them. From the iPhone Human Interface Guidelines:
Because users are accustomed to the
appearance and behavior of these
views, it’s important to use them
consistently and correctly in your
application.
If you change these sizes, not only might you make your application less usable, but that may violate Apple's Human Interface Guidelines, leading to a rejection of your application when submitted.