I would highly respectfully suggest to you to not simply run around in circles. That has been over done. Those types of layouts keep the average person's attention for a very brief period.
You have the kind of space most people can only dream of! Make the most of it. There are options other than large loops or circles of track.
Do NOT NOT NOT duck-under or crawl or climb on top of a layout. With your space, that is completely unnecessary and inconvenient.
With the kind of space you have, you can come up with a plan whereby you may access everything within easy reach, 24 inches or less. Your knees and back will love you for it. TRUST ME.
Take your time. Before you start laying a big circle / oval of track, you need to consider how you and others will be involved in this layout after its built.
You need to locate and carefully research model railroad DESIGN information; not just look at track arrangements.
The very most important thing to consider is PARTICIPATION. How will you and others have fun interacting with this creation once its completed? And, make sure everything is with 24" inches of reach. Down the road, with more experience, you will be glad you did this.
I should have added ensure your aisleways are 36 inches wide so people may get by one another.
What are your layout goals? What exactly do you want to do? How are YOU going to be involved and this is 50 plus years of experience telling you this: your answer should NOT be standing idly by looking doing nothing.
Who (name) is your layout? Where is your layout? What is going on on your layout?
I would recommend you consider constructing an operational (instead of a display) layout because that will provide you with hours and hours of potential fun. Display layouts are good for places where the public passes by and just want to see the trains roll.
Too many builders in our great hobby take away half the fun by eliminating the need for human interaction in a layout.
Sorry, I've given you way too much to think about.
I just hope you aren't another, like I was many years ago, who builds an-overly wide layout where track and switches are out of reach and there's nothing to do less watch. Then when you get bored, you've spent all that money and invested all that time, and are starting over from scratch.
Good luck and best wishes...read and research DESIGN prior to laying track. In the short term, lay some track on the floor and enjoy it until you come up with a good design plan.