Before you go ahead and replace your old double bed with the king size, it's important to consider how the bed with fit into your bedroom. And indeed, if you are buying a solid divan frame, whether or not it will fit up your stairs!
Large beds in small rooms can dominate the space too much. Likewise, small beds in large rooms can sometimes feel lost and look out of place.
A bed which takes up somewhere between 30-35% of your bedroom often looks best, although this is often quite difficult to achieve in practice depending on cupboards and other storage units. For a king size bed, this would mean a bedroom size of approximately 4.5m x 6m which would be a large bedroom and doesn't account for other bedroom furniture.
Sensibly, you should aim to have walking space of at least 70-80cm between the sides of the bed and the nearest wall, window or bedroom furniture. For reference, a standard corridor width in the UK is 90cm, so this will be plenty of space to move around your bed comfortably and enjoy your bedroom.