What size mounting bolt does a Shimano SL-M7000-11 take (pic included)

From Shimano's Technical Documents website comes this exploded view of your right shifter. Typically on these drawings, Shimano will give the size and length of any fasteners highlighted in the drawing (see the "Clamp Bolt" listing--#8--in the linked schematic), however the entire base cover with it's screws is one part number and no information is given about the screws themselves. As has been typical of these kinds of shifters over the years and different models, the bottom cover utilizes two and even 3 different size screws in one unit.

Your missing screw is part of the assembly listed as #3 (of the referenced exploded view) and carries part number, Y06M98030, the R.H. Base Cover Fixing Screw Unit. I googled, "Shimano part number Y06M98030" (without quotes in the search query), and this is the most relevant link that wasn't the same Shimano site I linked above. Still nothing about the size, but you can get a good look at the threads and the shoulder of it. My final answer is that it's an M3 about 5mm long. Because of that shoulder, a matching fastener won't be easy to find in the local hardware store, but I wouldn't be put off trying an M3 socket head cap screw, short as I could get (though a Dremel with a cut wheel could customize a longer one) and seeing if that would be a satisfactory substitute. Or, obviously, the part is for sale.