I have an 1870 home, and the roof sags a little in the middle. It wasn’t a major concern of mine when I bought it and still isn’t, but it’s something I thought I’d eventually look into. The rafters are about 4”x6” and spaced 24” OC so I’d say they’re properly sized. The thing I never looked at yet is whether or not the wall is pushing out in the center, the interior walls aren’t cracked but it’s all new drywall in that part of the house which I guess is a little suspicious. I’d assume though that considering the house is 150 years old, this might be normal due to some settling over the years.