Several modal windows show lists of elements, i.e. the list of events an operation will be applied too in the Actions->Start Task modal.
The height of the list is not limited - the modal window will just be high enough to be able to display the whole list.
That way, the height of modal windows is not limited, requiring the user to scroll down the "web site" to reach the buttons of the wizard.
But the Opencast UI is supposed to be a web application, not a web site...
I suggest to limit the height of enumerating UI elements within modals (e.g. lists of object) and use a scrollbar within that UI elements so that the modal window height stays the same.
Stay a web app (vs. web site)
No need to scroll down to reach the buttons at the bottom (unless you want to check the now scrollable list, of course)