As a general rule, we would usually recommend a lampshade that is approximately two – thirds of the height of the base you are intending to use. You would also want the diameter of the lampshade to be slightly larger than the widest part of your base.
Very generally speaking, lampshades that are 20cm – 35cm diameter work well for table lamps. Floor lamps usually work well with 40cm – 60cm diameter shades whilst an average sized room would usually suit a 30cm – 40cm diameter ceiling shade. There are definitely no hard and fast rules, it’s all down to personal taste and preference.