You might try 4'x6', landscape format. We typically start by centering the object around 5' above the floor. Might work better a little higher but you can try it out with tape.

Can't resist saying something about sliding rugs, they all slide , I have all wool Persian rugs , Afghani rugs and machine made rugs they all slide, you either put a really heavy table on it or you get non slip underlay . As for feather filled cushions , it again makes no difference cheep IKEA ones are actually better then Coco republic ones , they all slide on leather sofas.

if you want to be able to open the doors, load the boot and walk around i think you need at least 9m x 7m but if you to also want useable work benches and storage spaces (eg for the lawn mower etc) add at least 1 m to width and length

Um, why? Looks difficult to clean, difficult to access, and impossible to safely bath kids. I would avoid at all costs. No prospective real estate buyer ever said "I love the combined bath and shower arrangement" as they are generally universally hated, especially if there are toddlers, or elderly in the household . If you do go with this dangerous proposition then make sure you have plenty of well anchored grab rails so you don't knock your teeth out when your head hits the floor. Have a look at the Posh Domaine Shower Bath, which has a flatter, squarer base, rather than your traditional bath Best of luck, Dr Retro of Dr Retro House Calls and Dr Retro Virtual Visits.