I searched, and although I could not come up with a specific definition (e.g. half-way between the decking and the face of the insulation), "mid-height" is a commonly used term in architecture (means 'halfway', as one would guess.)
My guess is that it's halfway between the ridge and the top plate of the wall.
You may be wondering why the code uses mid-height instead of just plain height. I think it's because the width of the garage would also dictate the height. Using mid-height keeps the proportions uniform. (So a narrow garage doesn't have a high, steep roof - but a wide garage can have a high, low-pitch roof.)
Misc finds: (not much help actually...)
"1103.13 BUILDING, HEIGHT OF. "Height of a building" means the mean vertical distance from the established grade in front of the lot, or from the average natural grade at the building line, if higher, to the highest point in the coping of flat roofs, to the deck line of a mansard roof or to the mid-height of the highest gable or dormer in a pitched or hipped roof, or, if there are no gables or dormers, to the mid-height of a pitched or hipped roof."