1. What kind of design? A sexy girl tattoo should be something erotic, exotic, and even a little dangerous looking. What you should not get is something "cute", like a rose or some common type of girl tattoo. The design that stands out is going to be strong and sexy. You can still bring femininity into your tattoo by adding other elements around it. For instance a dragon tattoo on a female can be very sexy, while you can then add in surrounding stars or small flowers to accentuate feminine qualities, while remaining sexy and dangerous.
3. How large? Generally sexy female tattoo's tend to be bigger, which contributes to their strong, stand out look. Again, a small tattoo tends to be more "cute" than sexy. Whereas a larger tattoo stands out and can be much more elaborate, sexy and "dangerous looking". It need not be a large mural filling up your whole back, but an above average sized tattoo tends to be the sexiest.