Well it depends on how big your garage is ? Take a tape measure and measure the opening top to bottom side to side on the top to bottom add 3 ft that's how far back you will have to go, Keep in mind that today's code requirements for installations change every year The installer will have to re-frame the opening for the new door plus have to have room to attach on either side to the walls and attach the hard ware and motor to the ceiling. plus the actually opener on the wall next to you door into the garage. The less room they have the more you will pay.
If I have to navigate around your stuff not only do you risk damages Your also making it difficult to do the install which will cause me to be more careful, and that takes more time and wastes it too
If you don't leave enough room don't expect the installer to move your stuff its not their job and if you left stuff in the way its your problem if it get damaged. not theirs.They need room for the install if they wind up leaving and reschedule the work expect it, then clear the rest of the space Now a days you can rent a pod from a storage company that they will drop off and park in your driveway rent one for a week then put your stuff into it and lock it up that way it stays dry and not get damaged anything you leave in the garage that gets damaged is on you not them.
I did an install once told the people what to clear out they moved the stuff around the garage putting it in areas were I needed to install stuff, this went on for a few days up until I terminated the contract told them hire some one else. It was really frustrating trying to install a door and having to trip over crap that was all over the place then have to move stuff out of the way to do framing and bracing
I told them to rent a pod and they wanted me to pay for it, at that point I fired them, and moved on?
No one will install it if your stuff is in the way unless you sign a damage clause stating that anything you left in my way gets broken, I am not responsible You are, most people won't sign it Not to mention leaving something of value can result in theft the burden of proof is on you, Sorry for the long answer just trying to explain at different view points to help you understand. I have more then one door to do that day?
Hope that helps
worked in construction and they will need room to install power for the door I suggest you clear the whole Garage You will only have to do it once.