the recommendation for M-key is to not exceed 2.5 A, and that’s what you’ll want to adhere to. In case both sides of the equation are under your control and you’re creating your own pinout ...

Each keying defines a pinout for a group of interfaces like ... you won’t go wrong with getting a used M.2 B-key modem, typically coming in 3042 form-factorm, so 30 mm wide.