Need help with selecting correct Snowboard size?

ok so i was up in cascade for my thrid time and i rented a 156, i dont know if it was just the board or what but i just had no control over it at all and last time i rented a 155 and did perfectly fine, i really dont think that much of a difference could have made such a difference but it did. Im looking into buying a board and want to make sure i get the right size and width of the board, if i could please get some suggestions on board size i would appreciate it so much. I am 5-10/5-11 and just about 145 with size 10.5 boots, please help me out, thank you