Ok, so what is the actual difference in mm, or inches in between say a 42 and 43 size shoe. I know different manufactures sizes vary but is their a standard measurement in between sizes?

I bought a size 9.5 Adidas road shoe (wear size 10.5-11 sneakers) and they are a little too big. Now I have to decide if I want to go down to a 9 or 8.5. If I push my foot all the way to the front of the shoe I can stick my finger behind easily and If I force I can fit my thumb, this was without socks on.

Thanks,
Chris