I'm really tough to flatter, 'cause my internal bullshit detector is always going off.
If you tell me "oh, your cock is so big," or something along those lines, I'm wondering if you're just saying that to get some reaction.
But if you say, "god, I feel your big cock all the way up inside me" then I'm flattered AND turned on.
One of the first times we had sex doggy style, and my then-girlfriend (now wife) said "your nuts are banging against my clit and it feels so good" I almost came right there and then. 1) It meant I must have lower-hanging fruit than most guys, and 2) something I was doing was turning her on!