I guess Amtrak is really thinking those are "nuclear family bedrooms," and can't fathom of a Robert Heinlein-style family with 3 adults and 2 kids.

The bed lengths have been mentioned in this thread, but not the widths: the lower berth in both the regular bedroom and the family bedroom is 3'4" wide (the upper berths in both types of bedroom are 2'4", which is the same as the lower berth in the roomette). By way of comparison, a standard full-size mattress is 4'6" wide. I'd say two adults sharing a regular bedroom lower berth would have to be relatively petite and very friendly with each other, and two adults attempting to share an upper berth would have to be on their way to a contortionists' convention.