I just built a 24' x 48'. I had to go long and skinny due to the constraints of the building site, to fix the max. square footage that I could on the land available. I think, from a practical standpoint, having the building a little wider would have been nice- Originally I wanted a 30 x 40 but as the excavation progressed it became clear that wasn't going to work without spending a LOT more moving dirt.
One thing that I did, and it might help you, is I made a 3-d model of the building in sketchup, then put all the 'stuff' that I planned to have in the building in it to see how it fits. There are pre-made sketchup models out there for just about anything you could want, and the program is free and really easy to use. The model will let you play with layouts to see what fits and what doesnt, and will give you a great idea of how the space will 'feel' when done.
I've got two 'rows' inside- boats and truck, two deep, and there is PLENTY of room, but having the extra width makes the space more accesible than the depth does, at least in my opinion.
Consider bigger doors- too... I thought I was going to put a 10' wide door in, but changed to a 14' x 10' door, and I'm glad I've got the extra width. The additional cost was minimal (About $200), and well worth it.
I did 12' walls as well... Mine was 'pole barn' construction, and the builder did a full 12' under the trusses, which meant poles a bit longer that 12' from ground level, but again the cost up front for that was marginal as the posts were alreadly a bit on the long side and he had to trim them down. This way I have 12 clearance inside, rather than 11' 6" or so... I might put a lift in down the line and this will make a difference!
As far as location, Mine is set about 100' or so from the house. In some ways, it would be nice to be closer, especially on cold days, but being farther away keeps noise/smells/dust down at the house. Remember, though, you'll have to spend more $$$ to run utilities, etc. from the house, too, which might be a deciding factor if you are on a super tight budget...