Obviously what you can build inside will depend to a large extent on the space available. I've always thought I'd love to live in one of those stately homes which have a gallery. Some of these are 100 feet long - boy what a layout you could have there!
When thinking about a layout, there is a useful rough rule of thumb for a layout called the rule of thirds - the first third of the layout is the bit where the train is “coming into the layout”, the second third is where the train is “travelling in the layout”, and the final third is for the train to “be in the layout”. So for a layout to look good, and say it has a station at the right hand end, and a bridge at the left hand end, you need the distance from the bridge to the right hand end of the station to be at least 3 times the length of a typical train.
Here we need to first establish what kind of indoor layout would be acceptable. It may be that what you would like isn't really possible, so look at the alternatives. They get smaller as you go down the list.
You're going to need quite a lot of space, so let's look at the norms of each scale in terms of curve radius and circle diameter:
|Scale||Minimum radius curve||Circle diameter plus some scenic space|
Here we are going to need long trains, so let's look at how much space a short main line train of a loco and 8 coaches would take up
|Scale||length of a short express train|
Here the trains are much shorter, so again lets look at what length we need, and an overall layout length using the rule of thirds:
|Scale||length of a two coach train||3 times length||3 times length plus storage|
This could be a station to fiddle yard, or an industrial scene with off scene storage. There are very acceptable layouts in as little as 2 feet long all the way up to enormous!