To determine the correct length of the paddle, you can use the following metrics.

For constants:
Your height (the higher, the longer)
Board type/discipline (shorter for white water use, longer for flat water as you need more power to move forward)
Board volume (the bigger the longer, as more energy is needed to row forward)

For variables:
Wave height & frequency

(The more difficult the conditions are, the shorter the recommended length should be, usually shorten by 2-10 cm)

How to tell if the paddle is adjusted incorrectly
1. shoulder pain, back pain
2. awkward feeling when rowing
3. overstretching in the power phase
4. the paddle blade is not completely submerged in the water when rowing
5. loss of stability