There is a simple rule: paddle length equals your height plus 15 to 25 cm. You can refine this measurement afterwards, according to your paddling style. If you prefer stand up paddle surfing with smaller boards, the ideal size will be your height plus 15 to 18 cm. If you mostly ride or do short distances, add 18 to 22 cm to your height. Finally, if you like a bit of speed, race or do long distances, you will need to add 22 to 25 cm to your height.
A bit more on this
The longer the sup paddle, the easier to handle, especially in windy conditions.
The longer the paddle, the harder to paddle straight. It will also be heavier, but more stable.
If you combine several disciplines, it is advisable to have several paddles and several boards.