Im 5'6 is a 155cm snowboard too big?

Disregard what everyone else has said. That is the old school way of doing it and it is almost completely wrong. You want to base which size snowboard you get not only off of your weight and height but also your riding style. For example if you free ride then you want to have a longer board and if you ride freestyle then you want a smaller board. No one will be able to answer this question without you providing us with your weight as well as your riding style. If your looking to buy a burton board, they have there own board calculator which not only suggests a size but also a board at www.burton.com