How do I tell a guy his penis is too small?

Every time we have sex, I do not feel anything. He is starting to notice because I am silent during intercourse and I tend to check my email when we are in doggie style position. I do not want to humiliate him but I also do not wish to lie. He is amazing when it comes to eating me out and fingering my hole but I really need him to wear a penis extender to have fun during intercourse. How do I tell him this in a respectful manner without hurting his feelings? Cuz I ain't feeling it.

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