Resize image masks in Final Cut Pro 10.4

It's obviously possible to move (offset) the image mask around, but such a trivial thing as resizing it seems to be unavailable in FCP 10.4, unless I'm missing something painfully obvious?

The only solution appears to be to resize the mask in an external editor, guesstimating a right size, try it on, then repeat until you're happy or sick of it. This seems like a bit of a head-scratcher.

Plus, I can only choose an image mask from among the clips in the browser, which means I need to import the mask every time, which for me doesn't work unless I restart FCP. Importing the same image again doesn't have any effect, either using Cmd-I or using drag and drop from Finder, unless I restart FCP.

Something as patently simple as masking a clip with an image literally takes a couple of hours.

Does anyone have a solution?