Yes, I understand exactly where you’re coming from, but let’s not forget they’re not called fantasies for nothing.
I think perhaps when the feeling of suspended believe is broken, that’s when the author has simply gone too far. And let's face it, some writers are better at stretching it than others.
As for size, length, etc. I feel its more intriguing to let the reader visualize exactly how big, ‘big’ is, rather than giving precise measurements.
Alex.