No, at the moment you're pretty much stuck with the size you get given. I guess it's difficult to allow for different sizes when designing the sex animations. You'll notice that even when people have chosen different sizes from the menu for the height of their avatars, they'll switch to the average size for kissing, hugging and sex.
There are lots of hidden files in the resources sections, but there's no easy way to access them when in world. Even if you do, unless your partner has done the same they won't hear or see the same thing you do, as with custom clothing. Anyway, what do you need speed chat for? Use your imagination and your own words. It's far sexier, believe me!
Night xxx