You are going to spend a lot of time in this place. So its got to be a nice place. It just depends on your circumstances. It is, of course best its in your back yard, but, if not, choose one that is close to hand. My shed is on my property and was purpose built to house a boat. Not this boat but my previous one. Now that was, I suppose one of the limiting factors when making dimension decisions about the new boat. It had to fit in the shed.
Make sure you have some good strong beams overhead that you can use to lift things. We turned the hulls over in the shed and used the roof supports for the lift. No problems.
The ultimate, of course is a travelling crane, but I have never found one cheap enough - does not have to be motorised, I have got my inventing hat on as I need to lift the engines in (sometime around early 2009)
The front doors slide fully open and are 8.65m wide - great access. It gets very hot in Perth in the summer, so if you are building from scratch try and incorporate into your shed design a ventalation system that will create a through draft, that way when a breeze springs up you will get the full benefit. Rear roller door or lift up ventilating door would be the go I think
Good lighting and power points of course.