I hope your chair tucks away under the workbench or somewhere else so you have good access. I have got one of those exec type with high back and arms I use at the computer, the arms will not quite go under the desktop (worktop on kitchen base cupboards minus the plinth) so it protrudes more into the room when not in use, something to keep in mind with the tighter space in your potting shed.
Hope you didn't think I was implying that you was 'fat', 40" trousers mean you can't be more than 20" wide, which seems good to me