you need to creat some kind of cross flow. try to do it from ceiling down. Idealy you dont want the garage door open, but thats going to be your best route for now.
something I did:
on one side wall I placed 5 20"x20" box fans with filters to blow into the room for clean fresh air. On the opposit wall I used a Jenny's 20" explosion proof exhaust fan that I also built a filter rack for. I use this fan for the exhausting. Try to make sure that what ever you use to **** the paint out of the garage with is a sealed unit and can not start a fire witht eht fumes passing through it.
also I would suggest you set it up so that your paint overspray gets ****ed down to the ground. Meaning plumb the exhaust intake to the gorund area. have the intake fans close to the ceiling. Just make sure that all fans coming in or going out have filters on them.