iPhone 6 email max photo resolution
as far as I know, this is not an option.  

I just tried on my 4S w/iOS9. There's nothing in settings for Mail or Photos which addresses the default parameters for this check before sending a message:



This picture is from a similar thread where the author claims the "Actual Size" option is the default for image files under 500KB.

Per this 2009 article there used to be different options between "sharing" and "copy/pasting" pictures to an email, but I found in iOS9 that both methods exhibited the same behavior.