I thought of this when I was taking pics of a tape measure while testing various combinations of extension tubes.
Take a pic of a tape measure at the lens magnification (eg 1:1) with the required tubes in place. Copy this pic and paste it as a new layer on an *uncropped* pic of the bug. Reduce the opacity of the tape layer until both the bug and the tape are visible. Rotate the tape layer until it is aligned with the bug.
Here we have a small fly body length 3mm and body + wings 4mm. This can be reasonably accurate if the bug's body is parallel to the plane of the sensor.