11x14 because I can cut it down to make whatever I need. Paper cutter is set to 10" so I chop 14 to 10, then I have a 2 inch matt that I move to the spacer and make the 8".
No waste because the cutoffs are the test strips I run through the most important tones of the picture, say eyes and dark hair on a portrait.
With careful exposure and a contact sheet guide, I need few test sheets.
Another use is full format 35 mm down the center of 11x14. This leaves edge area to aid with mounting.
There are also templates to make smaller sizes from 11x14. Having them well marked allows me to resize under safe light conditions.
My 50+ year old Braumburger paper safe with 5 shelves helps me keep everything tidy.
It is 11x14 and has a roll up door similar to a roll top desk. Probably a garage sale item today like most quality darkroom equipment.
The paper cutter is also a quality one , not the chop lever variety. 35 mm enlargers are all Leica, 4x5 are Omega. Buy good stuff makes darkroom much more pleasant as does calibrating your exposure and development so you never end up doing 6 test prints trying to compensate for bad craftsmanship.
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