You can start by sketching out a 24" to 30" wide table around the walls of the room, with a drawbridge or drop down section at the door. You will also have to allow for access to the furnace area, which may affect the type of train table you design around the furnace. You may even be able to run a single main line track behind the furnace if there is room.
This train table layout phase will give you an idea to the isle width you will have, and the ability to reach all areas on the train table. This reach access is needed to build and maintain the layout, track structures, scenery, cleaning of the track and scenery, fixing derailments, etc.
This will also help determine if you will have room for a double main line, or possibly a twice around the room main line, which would give you a very long main line run.
Once you have a rough train table concept, you can start playing with the main line track plan. After that, you can look for options on where to put "the yard", industrial spurs and buildings/scenery.
