Can I just point out that no matter how big you think your shed is, it's never big enough.
At this stage you could still make it wider.
Also if you've got the land why not make it square ?
That way you can use all the same length wood.
I think it's just habit that makes people build them rectangular rather than square.
12 foot might seem quite wide, but once you've got shelves and machines, it soon shrinks to a narrow corridor down the middle.
I guess the roof might be a bit trickier, but you could always go for a slanted flat roof. After all you want something you can easily get the snow off.
Great news on the power too