You will need a very large yard for a great big room. Can you get permits and approval for it. Lots of practical things to think about. I would talk to planning and zoning before I got too deep into it. Do you have sewers or will you need a septic system or have it upgraded? I hate to be practical but, very important to look at these things.
I down size my stuff all the time. We are not doing anymore building and must live in our present space.