I have 10in twin peak bars currently on my 13 SG. The fairing mirrors are useless. What is the minimum size twin peaks or yaffee monkey bars to be able to use the fairing mirrors. Im 5'10". I was thinking 12in may work but not sure.

 

Put 10" yafees on my '15SGS. I'm 5'10ish. Fairing mirrors are almost useless. I can use the bottom half if I raise my elbows a hair. Works good enough for now till I figure out what I want to end up with. Thought about making a new mount to relocate the bat mirrors down an inch, but they have a funky profile to their backsides. Not a simple mod

 

Part of your answer depends on a number of things
The angle or pull-back you position the bars in - the further you push the bars away from your body towards the fairing will make the bars "taller", pulling them towards you have the opposite effect.
Then you have to take into account your arm length, are they straight or do they have a bend to them that might interfear with the fairing mirror.
Are you running the stock seat or an aftermarket seat that might raise/lower, mover you forward/away than the stock one.
Then lastly the bars themselves, you mentioned 12" Yaffee, these bars are actually closer to 14" in actual height when you measure them in person.
I have 12" Yaffe bars mounted on my 15' SGS and could not use the fairing mirrors, so I went with bar (mounted upside down) mirrors - yet others have had a different experience.
Bottom line, you will probably not know until you set your bars up and check for yourself.