Mine is 12m W x 7.5m D wth 4.2m gutter height. The extra depth is necessary, go the 12 x 6.25 and buy a parking hoist.

Currently mine has 3 x cars in it, a forklift and a 2 pole hoist plus heaps of cr@p, but I also have pallet racking down the centre and the RH side plus across the back. And a bathroom in one corner. And a mezzanine across the pallet racking.

Get a big wide door across one double bay, makes it easier to get stuff in and out.