The garage door is your big killer.

Uninsulated walls isolate much better than a bare metal big door, specially if it has other orientation than North.

Trying to isolate the garage door would be my first step.

After doing that, not touching the finished walls, I estimate around 0.50 to 0.75 Ton would suffice, depending on the orientation of the three walls, door and window, as well as other heat generating equipment in your garage-workshop.
Door or window facing South and big water heater inside may need as much as 1.0 Ton.

You can use one window unit or one mini-split system like Sanyo or Mitsubishi, installed as high as possible in the room.

You can PM me for a more accurate calculation when you move in and can measure things better.

Regards!