Thanks for looking at it Andrew.
In my opinion it's UX before aesthetic. I think users should not have to scroll when it's not necessary.
Thanks for the CSS I have forgotten it in the patch.
As you noticed I just increased the maximum size - it's currently fixed to a lower value. In my opinion having a maximum pixel size is wrong anyway. It already looks tiny on my computer, it will look even smaller on the futur ultra HD monitors/TVs. But this quick patch still being an improvement I thought I'll send it anyway
About aesthetic - some personal taste:
I would remove the top bar (title and cross). The user knows he's adding an activity, no need to take some space. In my opinion, it looks cleaner. The cross button is redundant with the Cancel button as the window is always on screen. The cross button has square corners which looks weird against the dialog rounded corners.
I'll wait for Barbara opinions before doing any changes.