The hit area for changing the device is 17px tall. Most other dev tools buttons are 26px. 17px feels really small, especially for a button I have to use every time I open RDM.
Thanks for the report, I agree it's small. At the same time, people seem to be sad about the amount of "wasted" height in the current design. Not sure where the right balance lies, so we may need some UX advice here.
I don't understand why there are three rows of stuff in RDM mode right now*, I think they could all be part of the top menu. * DPR and Touch are linked to the option in the device drop down so I don't understand why they're not next to that menu. And it annoys me that the device rotation button is always on the far right (when testing on my laptop with dev tools open I often have to scroll to reach this button)