My darkroom is 20 x 10 and I could still use extra length, the width is more than adequate. I have a wet side and a dry side with a 3ft aisle down the centre that saves a lot of walking back and forth for after I've done the work at the enlarger I just turn around and the wet bench is inches away. I would also encourage you to think carefully about the height of the wet bench. I decided the height of mine by working out the best height for me to lean comfortably on when I'm rocking trays. If you are over 6ft a low wet bench will cause backache. I also have a 2" x 2" leaning rail fixed along the full length of the wet bench and have found it very useful in preventing sore elbows. The roof does not have the traditional upside down " V " peak, the highest point is the dry side where I have my enlargers. if you have the traditional peak roof the highest point is on the gable end. Just a few points that you may have already considered.