Its only possible to start a measure of a mine field, when I draw at least a tiny selection rectangle with the mouse.
A simple mouse klick does not work.
This also makes it impossible to measure a mine fields from the map view (m).
When you try to "select" a mine field to start a measure, you will only drag the map view around.

If you have radar coverage and zoom in to the terrain, dragging should work just fine (just tested on a fresh Factorio).

I'll gladly accept a pull request to add the ability to tag a site from map view, but it seems to me that this workaround should suffice (since I imagine most players have the necessary radar coverage -- for remote blueprint capability, if nothing else).