The common feeling around here is 4 sq ft per chicken indoors plus 6-10+ sq ft per chicken in the run. For five chickens, that would be 20 sq ft indoors (like, a 4x5' coop or whatever) plus a run of 5x10' or so.
BUT, please be aware that these numbers are largely pulled out of the sky and time-tested only in the sense of not having consistantly catastrophic results or anything like that
Many of us would argue that more space is a lot better.
Personally I do not feel comfortable keeping my chickens in less than about 10 sq ft per chicken (MINIMUM) *indoors*, and a run as big as I can stand to build it. *I* think they are much happier that way - everyone's entitled to their opinion and that's mine.
In a cold climate I would have solid, probably insulated walls and ceiling for an in-garage coop (unless your garage is heated to at least 32 F or something weird like that
). But I would also have windows for light/observation and VENTILATION, both to the outdoors (thru exterior wall) AND to the indoors of the garage. You will probably want to make extra sure you have good ventilation in this situation (yes, even in midwinter up north!!) not just for the chickens' health, which is the 'usual' issue, but also so as not to stench up your garage more than necessary
Be aware that chickens create dust -- both bedding dust and a fine white greasy 'feather dust' when they are molting -- in case you have high white-glove standards for your garage stuff <g>
Best way to keep down odor is to keep the area well-ventilated indoors and DRY (as possible) outdoors, and practice good sanitation i.e. clean things out regularly. You might consider a roofed run (the sides would still be mesh and thus plenty o' airflow) with a sand or gravel or mixed footing, all of which will encourage dryness.
Good luck and have fun,
Pat
