It depends what the market standard is.
I do older homes (older for my area, circa 1900s) and I couldn't fit my car into their 1-car garage (which was added after the home was constructed). However, many of these owners buy very small cars (especially in San Francisco).
It sounds to me like it is a garage, but is limited in its utility. The question is this: Does that limitation effectively eliminate the garage-utility expectation in the market where it exists?
If so, then I'd say you have a garage that is not functional, and can be used for storage. I'd probably want to find homes without garages to use as comparables.
If the size doesn't eliminate the garage-utility, but limits it, then your garage probably contributes less value than a full-size garage. What that difference may be will be challenging, but not impossible, to reasonably estimate.
A house with a 2-car garage vs. a house with no garage, all other things being equal, may sell for $20k more.
A house with a 2-car garage with limited utility would reasonably be expected to contribute value less than $20k but more than zero.
Research (and, I'd recommend calling agents and getting their feedback) will help you narrow the range, and then pick your number.
(I'm assuming it makes no sense to revert the garage back to its original depth).
Good luck!