Thank you very much for the photo, mine will be in same place against garage wall like yours, probably why it's similar in length. You've even got same garage door as me too.
Although I'll be using it I am making it for my daughter to have fun with and play with, the operating space in the middle is great idea and she can stand on something to be able to play with the trains and layout, like you say 00 is maybe more child friendly, I can always do an n guage layout later in future when she is older. Yup decided on plywood, my father in law is going to help build the baseboard table. Will have a think on the height of it. As I've only got a Mac I will have a play around in Railmoddler now I know my space.
This leads me to my next question on how to control it, DCC or DC, what's your thoughts?