I would guess the drawers are something like 120 x 70 x 40mm
This looks identical to a couple of units I got from Lidl a while back and have on my garage wall (sorry). You were pretty close - specifically, the interior space of a drawer is 65mm wide, 115mm from front to back, and 35mm deep from top to bottom. The rest of the space is taken up by runners and the little drawer-pull-slash-label-slot thing at the front and clearance and so on. Also there are two points to mount little slide-in plastic dividers on each side of the drawer which stick a millimetre or two into the drawer space, which is fine for cubes and stand-ups and so on but might be annoying if you were using it to store your collection of 115mm-long wooden sticks or something.