You have two alternatives:
1. You can choose the ‘Responsive’ option, from the ‘Choose a size’ menu, that will adapt the search box to any screen size, automatically.
2. Otherwise, you could choose one of the sizes available. These have been designed to the optimal width-height ratio.
The perfect size should make the Search Box easily visible without interfering with the user experience. This involves leaving enough space around the product in order not to be confused with the information around, and to keep the viewer focused on the main content.