Size matters. As trivial as it sounds, this is the first important decision you must make, affecting both your sleep and your wallet. Having enough room to be comfortable and undisturbed is essential for deep, restorative sleep. If you are a light sleeper, and easily wake up to the movements of you partner, consider a larger bed where less motion will be transferred if you are further apart.
Bigger mattress will cost you more money. On average a king or california king mattress will be at least 30% more expensive than the same mattress in queen size. That is a significant amount when considering a premium product.
And don't forget to account for the additional cost of accessories. If you decide to change the size of your mattress you will need to upgrade your pillows, sheets and mattress protector at minimum. You might think about getting a new bedframe, headboard or whole new bedroom set.