At what size or fit do you guys think a boot is to big to work with. ie 3-4 time a year skier type 5-6. Mid level boot.
difficult one to say exactly as it depends a little on foot volume verses shell volume but generally much over 25mm is kind of pointless, you may get the boots better but from past experience people don't just want the boots better, and also form past experience as soon as a fitter makes an adjustment it appears that the boot becomes their problem to make perfect, even though they didn't sell it in the fist place (hence the reason for not working on boots which are way too big)
Not to be smart but don't understand the question exactly, but I think you are asking "How much to big can a boot be and still be acceptable". Here there is zero tolerance. Almost any boot pain or performance problem you can list can be caused by boots that are too large so we don't fight it. If there are complaints and the boot is large we replace them.
Impossible to describe what is too big as there are volume issues to address and not only length. But since length is an easy starting point-one size too big is too big
then add to all the above the tolerances of each individual skier, yesterday i saw a client who had a foot that looked like it have been run over by an Abrams A1...... one foot measured 29 (just) the other measured 30.5 now in the 29.5 shell we ended up in some might say the larger foot was too big for the boot with around 10mm of shell check (a good performance fit), but the smaller foot had 20+ mm so would be considered too loose, some judicious padding and stretching the shell for the collapsed foot and all was well
some times you just have to compromise, but in most cases too big is simply too big