since you are using a fairly small tank, you can get by with a changing bag of roughly 14"x17". However, a somewhat larger bag is more comfortable. If you intend at some point to use large format, then consider a changing room, which looks like a miniature tent. These are far more comfortable for any size film, but much more pleasant for 4x5 and up. Also, with a changing room you are less likely to have problems with dust and lint.(posted 8218 days ago)