I just reconfigured the front of my shop. The building is 26x48. The 48' wall is the front.
After much debate and planning I decided on one 10x9 door and one 18x9 door. I've got a Dodge 3500 that gets parked behind the 10x9 and I would want a door any smaller than that to drive a full size truck through, but this door works great.
The 18x9 door is nice in the fact that I can easily pull two vehicles in and out of the building without feeling crowded. Its also nice that I can pull things in side ways.
In short it a personal decision. How are you going to use the space? If everything is going to be driven straight in 10' doors are great, if you will ever need to back a boat in or park something cross ways, an 18' door would be nice. Lay it out a paper with what you will be pulling in and out of the building and it will all become clear.
Dave