They get between 2 and maybe 2 1/2 inches, with females being a little bigger.
I give mine stars all of the time so they can eat when they want, but I have seen many posts of people who feed only every few weeks and have kept them for long periods and have no health issues. I base my feeding schedule on the fact that I have never seen one in the ocean without a star. This could be because they always have one available, or they are harder for me to find without the star and I just don't ever find them when they are not feeding, I'm not sure.
I feed a single star to a pair and they eat it in 3 to 7 days depending on it's size.
You could cut legs up and feed them, but they do not (at least any I've kept) eat dead stars, so frozen probably wouldn't work. Mine abandon the stars as soon as they die, and then I remove it, though most of the time they eat it down to nothing.
Males and females have different color patterns under the body, I don't have a photo, but it is easy to tell so if you look just get two that have different color patterns on the finnetts under the body and you'll have a pr, plus you can look up the difference online and see photos and see for yourself.