What length of skirt are women wearing?

Just below knee seems to be the trend. I saw some very nice summer dresses on some girls.

Are sandals acceptable? I will be taking many skirts and capri pants with me and have some very comfortable dressier sandals that I would love to wear.

Even this man who does not try to look fashionable can answer the sandals question. At one level -- that of those who are fairly unconcerned about appearances -- they are acceptable without reservation. If you care about appearances, they are acceptable provided that they do not look ridiculous with the rest of your ensemble. Think brown chunky walking sandals with a neat black skirt and pastel blouse combination, or think gold-sequinned wedges with khaki capris and blue-and-white striped T-shirt. Wear them if you like. You might turn some heads.

Wear whatever you want, but err on the side of conservative, unless you don't mind some stares. I've seen 50 year old local women wear short shorts in Prague (with 4" heels), but their bodies were extremely tight & in shape (I don't think my body was ever that nice, even when I was 18!!) So, it's all in persepctive. If you look ridiculous in it in the US, or wherever you're from, you'll look extra ridiculous in Europe.

PS - bring dress clothes for concerts & a jacket for evenings. It can go down to 50 at night, easily. With that said, bring pants instead of shorts, since it is usually cooler than warmer at night, and you don't want to overpack...