His wife said he was something like 10.75".
At the start, Holmes was widely publicized as having a penis ranging from 13.5 inches to 15 inches, depending on which ad copy a particular adult film distributor chose to use. In reality, according to those who performed with him on screen, Holmes' actual maximum length when erect was 12.5 inches. It has been noted by several porn historians that the advertising of Holmes' endowment finally settled on 12.5 inches for two reasons:
# Holmes, who was 6'2" tall, started his career doing 8mm "porn loops" with female performers who were, on the average, no taller than 5', and were of extremely slender build. However, due to clever camera angles, most of these short women appeared to be ~5'7" tall, which based on scaling made Holmes appear as if he were actually between 13 and 15". His length as well as his 7.5" circumference at the base, and 8" at the head when fully erect by most accounts resulted in quite a few porn actresses refusing to work with him, with "self-preservation" being the primary justification. Holmes was viewed by most porn actresses as being a "real gentleman", even when obviously intoxicated on heroin. However, after one reported instance where a 4'9" performer suffered internal hemorrhaging after losing her balance and receiving Holmes' penis far harder and further than expected. Word spread through the industry that if a film starring Holmes made a casting call, shorter women need not apply.
# In 1978, several companies were licensed to produce a dildo that was cast from Holmes' erect penis. While this allowed people to experience Holmes' manhood firsthand, the dildo was only 12.5" long. After the dildo was released, according to those working in the porn industry in California at the time, if the dildo size was accurate, then the descriptions of Holmes' being longer than 12.5" were clearly a case of false advertising, and depending on which measurement was correct, someone could easily get sued. Shortly afterwards, Holmes was officially listed as being only 12.5" long, where he stayed at for the remainder of his life and career.
-AllExperts
This link has photos that compared his weigh and the size of the girl and all this other stuff to determine his size.
Here
And look at the pictures along the side, its not hard to see how people could think of him as being bigger than 10 inches or so.
But sadly enough, John Holmes died of AIDs on March 13, 1988.