Since you are not sure how big your building will be, it's probably safest to plan on putting the big door in the center. More importantly is that you make the door big enough to get in and out of easily. Mine is ten feet wide and I should have gone 12 wide. Having it centered allows you to maximize our storage on one wall and have a work bench on the other wall.
My plan for my next shop is to start out building the main barn as wide as I can, but with super tall walls. I'm thinking 16 feet, but we'll see what happens when I actually commit to doing this. I will place the building so that I have space on both sides of it to add on to it. I want the tall walls so I can easily attach the lean to, or additional barn to it and still have a decent sloped pitch to the roof. Eventually this enclosed lean too area will be where my dedicated wood shop is, which would be separate from the tractor storage, vehicle fixing area.
I want to be able to build it, then add on to it, and then add on to it again and again if I need to, and as I can afford it.
My starting out dimensions are probably in the 30 foot wide range, and as long as I can possible afford to make it. Probably 60 feet long. 30 feet gives you plenty of room to have a vehicle inside and still be able to get around it with a work bench on one wall and a ton of storage on the other wall. I also like the idea of having a door at either end of the building so I can enter it from either side.
Currently I'm at 24 wide and 30 deep with 12 foot tall walls. It's more of an over sized garage then a workshop. I can park my truck in there, or my tractor, and work on it, but over the last few years, it's becoming more and more difficult to squeeze everything into there.