Personally I'd brake the map up into small sections. Each section is loaded when the player gets close otherwise your loading the whole map to memory which is very hard going. Your fps going to have a hard time when you and models and everything else.
The scale is up to you play around with the size see which fits best for you. If its a town house and it looks like a skyscraper then it needs scaling down.
Its a good idea to write a basic movement program and add the building. That way your seeing it from the players view. If the building looks too large or too small from that angle you know it need a fix.
I'll add something later on.