Speaking as a petite female myself, I've never had any issues with the size of the dog(s) I walk. I've walked 4 at once, the smallest being 68 lbs., the largest being about 90 lbs. I currently have 3 boys - a 85 lbs., 65 lbs., and 60 lbs. The 60 pound Galgo is the one who knocked me on my butt going after a squirrel, but only because he saw it before I did and I wasn't prepared. I'm partial to the big goofy boys as my experience has been they are more laid back. I've fostered and dog-sat for females, and unless it's an exceptionally easy female, I'm going to stay with the big boys.
As long as your wife is prepared and keeps watch for situations which may arise on a walk (cat, squirrel, groundhog, etc.), she should be fine.