The Enrol users dialog needs to be full screen. This is a similar requirement to the filepicker and activitychooser as documented in MDL-39851

Steps to replicate:
1. Access a course using a mobile device
2. Navigate to: Course administration -> Users -> Enrolled users
3. Click the 'Enrol users' button

Expected outcome:
The 'Enrol users' dialog is full screen, with all controls contained within it visible.

Actual outcome:
The dialog is not full screen, and so controls are cut off. It is necessary to move the dialog, a fiddly operation on a mobile device, to access the required controls.

Additionally on iOS the automatic selection of the 'Users roles' drop down causes the standard select UI to be displayed, further obscuring the other controls on the dialog.

I've attached screenshots to show you what I mean.