Okay, in case anyone needs it I randomly found a nut searching in my drawers and it's the perfect side for my M.2 standoffs, the size is standard M2 nut (they have a diameter of 2mm, unlike M3, M4 or others). I'll have to but standoffs with a little bit larger screws tho, as the ones I have are too short to go through the motherboard and attach them correctly.