The size of your down duvet depends on your mattress, your body size and your bed. Down duvets come in many different sizes with a width of 135 / 155 / 200 / 240 / 260 x length 200 / 220 / 240 cm.
In Germany, the standard size for double duvets is 200 x 200 cm. This size is sufficient for people up to 1.80 m tall. If you are taller, it is better to choose a duvet with a length of 220 cm. The same applies if your mattress is 220 cm long.
For a standard single duvet, the size of the down duvet is 135 x 200 cm. If you like it a bit cosier or if you are particularly tall, you should choose a down duvet 155 x 220 cm. Down duvets for babies are a special case - they are correspondingly smaller.