First, more important than welcoming you to the forum, I get dibs on the Irish stuff if you sell up...
Welcome to the forum!
10 x 10 is a huge space. You could literally run two different levels, and have both levels criss-cross the shed several times. At a guess, you could have two "peninsulas", one at each side, which would give four runs the length of the shed. If it were possible to squeeze in a fifth run, you could connect to the start of the first with a diagonal running above the rest of the track.
As DC says, and as Noel pointed out to me previously, think long term and make your baseboards small enough so that they can easily be transported in the future. I'm not sure what fits comfortably across the back seats of your average car? Maybe 1000mm or 1200mm?
Are you running DC control or DCC control? DCC means you can fit a lot more in as there's less need for complete loops. I've recently bought a DC Gaugemaster Shuttle controller to try on an end-to-end DC layout, but have no feedback to offer as of yet.
Best of luck.