Im waiting to fit mine back on (tho still have the screw). The screw doesnt have any markings on to give you a size of screw or thread.
As a rough guide - the allen key you will need is a fair bit smaller than your bog standard IKEA allen key tho I dont know the size of the actual one needed. The actual screw is 13 mm long, of which 10mm is the thread. The threaded part is 3mm wide.
I reckon that if you can get a few different sizes of square ended (not pointed) screws to try, you wouldnt have too much trouble to find one that fits.
The above advice about grinding or sawing an allen key to avoid having to get a new glove box sounds like a good idea.