Let me offer you a practical concern, as well as encouragement to go ahead and indulge yourself, with planning and intelligence; plus, enthusiasm.
You may want to consider building a layout that has every inch of it within arm's-reach of your standing in the aisle(s). I'd also keep crawling around on my hands-&-knees to a minimum. Plan carefully and cautiously.
I offer that advice because, at 73, I am not looking for reasons to mount the layout any more - climbing aboard, especially at this spot which is where I usually used to climbed aboard....
....putting a ladder by that pole and... ....stepping over those bridges, into the valley of the first level, with the second and third levels around me...
....both within reach, when needed (derailments; dusting; changing trains on the upper levels, etc.) I had thought about going up, a couple days ago, but decided it was not sufficiently necessary to do so.
Also, you can always start with one size; then, expand it later if your imagination requires it and your energy permits it. I did. Those two sections of the layout on the left and right of the top photo were not part of the original plan. They were added later once I had what had been planned accomplished and running.
That's the best perspective I can offer you, from personal experience, Dennis.
FrankM