Trudi,
I'm not sure, honestly. I know many sites using RocketTheme's RokBox module with our Login module, but haven't heard of that issue. I don't know a good answer to that question though, as it seems like something the RocketTheme guys could probably answer better. From your capture, you may be able to add:
.rokbox-center {height: 270px !important;}
However, that may completely break the 'growing' effect, or be overridden by the Javascript itself that's creating the popup. It's our best guess though. We're not template designers, CSS experts, or the creator of RokBox, so wouldn't really know how else to modify that.
Hope that at least helps get you started!
Alex