Walls and Ceilings - Garage Ceiling Joist & Dry Wall Advice

My wife and I just moved into our new house. The garage is an older 2 car garage (early 1950s) that we would like to remodel. It currently does not have any drywall, so the ceiling joists and framing is exposed. We would like to add drywall to the walls and ceiling. My contractor told me that the current ceiling joists are not adequate to support the load of the drywall and would need to be upgraded. The width of the garage is about 22 feet. I believe the contractor said that we would need to have at a minimum 22 feet length 4x8s running across the ceiling, and if we want to use the area above the ceiling joist for storage space or to support a person, we would need 4x12s.

We would appreciate it if someone can answer the following questions for us:

1. Is the contractor's recommendation correct about the size of the ceiling joists needed to support the drywall as well as to support additional load if we use the area above the ceiling joist as a storage area?

2. How long would a job like this take? I believe the work that would need to be done is to remove the current ceiling joists, install the new ceiling joist, dismantle the garage door opener, install/tape/paint the drywall, and reinstall the garage door opener. My contractor told me that he would need about 3-4 weeks of work. Does this sound about right?

Thanks,
Michael

<[QUOTE=mchou]that he would need about 3-4 weeks of work.>
Even working alone, thats a little long.

What is the style and pitch of the roof.

<I believe the work that would need to be done is to remove the current ceiling joists.>

Shouldn't be neccessary.

<Install the new ceiling joists.>
OK.

<Dismantle the garage door opener.>
No. Disconnect brackets and support operator on ladder, hang lid (ceiling) and reinstall, hang walls, tape, sand and paint.