You can't resize the popup window from the Options page (adding that requires fiddly little boxes) but you can do it from the popup. Click the double-headed arrow button at the upper right, select the lower option to use the current window size, then resize the window, then finally click Save. If the arrow button doesn't respond, make sure the extension updated to version 1.7.1 because there was a bug in version 1.7.0 that prevented it from showing the form correctly.
Update: In version 1.8, I added a note on the Options page explaining this. Thanks for mentioning it.
