I have been tempted to build a raised platform at the rear wall of my garage too,tall enough off the floor so my tractors can be parked under it without taking my head off,or my truck's bed or hood will fit under it--so I can free up more floor space...probably have a few stairs on one or each side ,so I can walk up onto the platform..
I have some 6x6 posts I was going to use for legs,and I would tie them together with an "X" brace if the platform wasn't anchored to the floor,so it cant tip over if an unbalanced load was placed upon it..I'd use 2x6's for the "joists" and put them on 12" centers,then you wont ever have any issues with overloading it...and use metal joist hangers,not just nail into the end grain too..
At one time I had enough pallet racking left over I could have used,that was 10 feet wide,but it was only 24" or so "deep",and I'd need more space than that...it'd be a lot easier to use the racking though,than stick build it from wood...
I may wait and see if I can score some more racking,every so often I see some at a local junkyard or on craigslist...
If I doubled them up,so I had a 10 foot by 4 foot platform that would be better,and I wouldn't need to anchor them to the floor,and a sheet of plywood will fit without cutting it lengthwise...they would even be strong enough to put a few tractors up on the platform too,but I doubt I'd go that far,its pretty tricky trying to get one up off the floor that high...my friend had a setup like that and used a winch to pull his "extra" tractor up onto the platform,but it proved to be a pain,every time you wanted to use one of them...