If you are short and want to wear a skirt/dress and boots, you should wear a miniskirt with dark, opaque tights and dark boots. The monochromatic color scheme is elongating. If you want to go bare legged make sure the top is conservatively cut...long sleeves, high neckline. If you're tall and want to wear a skirt/dress and boots, WhoWhatWear suggests you wear a skirt that hits just above the knee, especially if you're going bare legged. It's all about proportion--you don't want to show more bare leg than skirt. Unless you're wearing tights, skip the miniskirt because tall plus mini can equal hoochie hoochie.
When wearing jeans:
If you're going to wear flat boots, you should wear skinny jeans tucked inside your boots. Do not wear them over your flat boots; this is a look reserved for boots with heels. If you are short and worry that tucking jeans into boots will make you look even shorter, you should wear a longer top(anything that hits a few inches below your hipbones). This will give the impression of a longer torso. If you're curvy, it's best to stick to dark denim or black jeans. If you're tall, you are lucky and pretty much anything goes.
Sources: Altamira NYC, Sally Jane Vintage, WhoWhatWear