That really depends how you consider passenger cabin sizes.
Economy-class cabins carry one passenger per two cargo pods for the same internal size (or per three cargo pods on the optimised Panther racks) which even assuming that the Panther racks pack the cargo in as densely as possible [1] with little spare space gives each person at least 6 cubic metres of space (which is considerably more than you'd get in most "economy-class" mass transport today).
[1] i.e. ignoring the much larger volume requirements of fighter hangars or SRV bays.
The way I look at it, a given passenger cabin can only be the same dimensions as an equivalent cargo rack (it could be smaller, but not bigger) because it's got to fit in the same slot as a cargo rack.
So, if you look at, for example, a 6E cargo rack, it can hold 64t of stuff.
Again, using water as a benchmark, if you're carrying 64t of water the cargo rack needs to have a volume of
at least 64m³.
A 6E passenger cabin can carry 32 people.
If the 6E passenger cabin takes up 64m³, it can be 2m tall, 8m long and 4m wide.
That's actually quite a decent size for a cabin, and could probably fit bunks, seating, bathroom facilities etc.
Having said that,
that isn't an especially efficient way to carry slaves either.
Take a room 8m long and 4m wide and you could probably stick bench-seating in it and carry 100 people.
Dunno if anybody at FDev actually thought about this (rather than just arbitrarily creating the stat's for canisters, escape pods, cargo racks and cabins) but it
seems like there's a common theme of allocating people around 2m³ each.