There are 4 doors into our shop.  The bifold that is 18' tall and 35' wide.  A roll up door that is 18' tall and 24' wide to get most planters and cultivators and large tractors in.  It also opens much quicker then the bifold or the hydroswings everyone loves here.  This is the one that I bring in the heads on trailers.  Then there is the 12x12 for pickups, skidsteers, lawn mowers, Rangers, 4 wheelers, andtractors with no duals like the 7410, 1086, MX125, etc.  A door like that gets used a bunch here.  Then I put a 10x10 to get from the cold storage to the shop.  I just wanted a door big enough for the skidsteer to go in and out, and be able to put stands, pallets and such in the cold storage from the shop.  The 1086 will go through that door, with a 280 bushel wagon or the feed grinder.  Its tight, but works.  The cold storage has its own 18' tall by 24' wide door to the outside.

I agree, a 12' door is too small for many machines now, but if you are putting in a 20' tall by 60' wide hydroswing(or whatever), then you should really consider putting a 12x12 so you don't have to open 1/2 of your shop wall in the winter time to run in a 4 wheeler to fix a flat tire.

Another thing to remember is when you put in a huge door it does take away wall space to be parking welders, work benches, band saws, tool boxes, drill presses and such.  I drew the line at 35', and low and behold we bought a 35' head.  But I set up the shop space so even if we bought a 60' head, we could get it inside and hang it on the combine.  Our combine never has the head on it unless its in the field anyway, and the shop is setting on a 40 acre farm.  If the combine breaks down bad enough to come to the shop, we will end up putting the head on the trailer to get it home anyway.

Michael