As taken from the Shells FAQ thread: "Each shell is a private space, larger than a Relto island". (here is a
size comparison example ). Shells are around 180 feet accross inside, so that's large enough to put in a very nice house. (or anything you want actually)
does it matter what you put in a shell or does it have to be the same as the type of shell it is. Like commercial shell having a personal house in it?
Honnestly this hasn't been discussed in a long time.. Originally 'commercial shells' were made for people who want to display their work and 'sell' it to other people. For instance you are a 3D artist who has made some nice 3D plants you display them in your shells so people can see them and then they'd contact you to get them.. But again this was not very clear and it hasn't been mentionned by anyone in a long time.
As far as I am concerned if you want to get shell 428 and put in something 'non-commercial' that's fine with me..