I just recently "finished" my new darkroom. ( Finished is in quotes because it never will be!! ;-) ) The room is under the house. The house is on a hill side. I excavated nearly 40 yards of mostly Bay Area sandstone. The darkroom floor space is 18 feet long, 8 feet wide with a storage area that is 15X8 adjacent. (Storage space = "wife acceptance factor")
Due to the slope and my existing foundation, I cantilevered the wet space counters and sink an 3 additional feet. So effectively, the width is 11 feet. It is comfortable, but if I could have gained more width I would have, even just an additional foot.
If you have the space available, ( it sounds as if you do!) Build as big as you can. This is the cheapest and easiest time to go large. Bear in mind that framing is generally based on 48" widths. (Studs 3X16" OC) Interior sheathing, drywall, panelling or plywood all come in 48" widths. So 12X12, other than additional foundation costs, is probably, in the overall cost, the same as 10X12. The least expensive square footage is at the framing & foundation stage. Unfortunately if you do not, I am sure the time will come when you sincerely wish you had!!
Best of luck and keep us posted.