If you don't dare go for a mini, knee-length is ideal.
Wearing them over trousers can make you look heavy. Go for a dressed-down look by pairing a poplin bubble with boots. But if you're wearing thick, heavy boots like the ones in the photo, go for a slightly less voluminous skirt that just skims your body: again, the knee-length bubble can make you look heavier than you are if you're not careful.
For a more elegant look, take a tip from the haute couture designers: the post-war look is very popular at YSL and Lanvin right now. Go for a heavy skirt made of good quality material in a perfect fit, match with court shoes and scrape your hair up into a bun or even go for a wide-brimmed hat.
Photo: Poplin bubble skirt, Somewhere