My contractor wants to cut a 4"x5' access slot through the 2x8 board that separates the bottom of the garage wall from the very top of the basement so that he can get drywall downstairs more easily. This seems like a really bad idea to me as it could compromise the integrity of that wall (I assume that wall is load bearing since it runs all the way to the roof). Any opinions?

(I assume that wall is load bearing since it runs all the way to the roof). Not necessarily so. Bearing walls are those parallel to the roof run. In a standard gable roof, only 2 outside walls are load bearing, maybe 1 inside depending on truss or non truss construction. 2X8 would be small for a rim joist which this appears to be. However, I would not allow them to cut the 2X8 board. No basement access door?

thanks! There is a basement access door but I think they want to use 12ft drywall sections and the turn is tight. The board may be larger than 2x8... I was just estimating, but in any case it seems that the risk is all on me just to make it easier for him.

Garage to basement, that is by code is considered a firewall and not the same as interior walls. I can't give a qualified response as to how that will or if it will compromise the fire resistant properties which is what I would be concerned with..

He can use 4 x 8, good grief.
I'd be concerned now that he knows what he's doing finishing a basement. There is a lot to know, some of it new, regarding long term moisture problems, no matter how slight. You can't just put a stud wall against a below ground foundation.

Appreciate the concern but he has been doing basements for a couple of decades. Just trying to avoid extra taping and mud with smaller drywall pieces.

I've seen people doing work for decades that really didn't know what they were not all that great a what they were doing, or took illegal shortcuts to pad their profits. Particularly if he was the low bidder when everyone else was much higher.