People have pointed out there are cultural considerations in defining what is modest. Well, the example used most that I have seen is people in Africa that wear very little even when they go to Mass because that's what they wear, is not immodest there but would be very immodest to us in the developed world. Besides that, I don't know of other places where the normal fashion would be immodest to us.
People also point out environment circumstances, too. Obviously, beach/pool vs. not beach/pool. The summer when it is hot outside.
One reason why it is important to dress modestly is to not cause scandal and tempt another to sin through objectifying you or imagining you as only something to use.
I think it is easy to agree on some clothing that is always immodest: Yoga pants/leggings all exposed, bikini, speedo for men, extremely short shorts for women (men don't wear them as far as I know), skintight shirts/tops, and cleavage revealing shirts/dresses.
Now for questions for the community to answer, and if you have reasons, I hope you include WHY you draw the lines where you do.
1. Where do you draw the line to say that women's shorts or dress/skirt length are modest and not immodest?
2. If a swimsuit is considered modest, does clothing covering an equal amount of skin in a different environment than the beach/pool also deserve to be called modest, even if it's exposing a lot of legs and shoulders?
3. Assuming we agree revealing cleavage is immodest, how much, if any, of a woman's shoulders/neck may be exposed and still be called modest? Does it make a difference if it is a fancy dress or a tank top or spaghetti strap top?
4. Jeans: they cover the legs entirely. There are definitely super tight jeans that would be immodest like yoga pants. Yet most jeans for women are not super tight, but still are form-fitting without any bagginess. Do you think the normal jeans and other form-fitting, not-baggy pants that women wear are appropriate?
I look forward to hearing what you fellow Catholics think! As of now, I myself answer my own questions as follows. Still, others might be able to persuade me to shift my views.
1. I am actually undecided about shorts. Too short is too short, but I also have the thought, "What's so bad about exposing a little more leg, something both men and women have and know how it looks?" The middle thigh is just like the lower thigh, just higher. On the other hand, I think skirts should at least drop down to the top of the knees.
2. No, an equal amount of clothing is not modest in a different environment because environment does matter.
3. I am undecided. My friend that I think cares about modesty wore a dress that exposed half of her shoulders but was not revealing, and open in back to a little below her shoulder blades, and I thought it was modest enough and she looked elegant.
4. I think they are okay if they are not super skintight because they have become so common. They don't stand out like tighter pants do.