Just tried this calculator with your specs: Snowboard Sizing Guide, Size Calculator
While no calculator is 100% accurate I find this one actually pretty close most of the time.
It says you should be on a 160-162 with about a 258mm waist width. The trouble with you is you're very light for your height and shoe size. Shit I'm 10 pounds heavier than you, almost a foot shorter, with size 8.5 feet! Since it's your weight that will be flexing the board, all boards will feel stiffer to you than they should for a "typical" rider.
So for a first board I'd be looking in the 158-162 range, make sure it's wide enough for your boots, and make sure you get properly fitting boots. I wear size 8.5 shoes but have 7.5 boots. They need to be fairly snug.
A shorter board won't really hurt you, and in fact may help while you're learning. I was on a 153 until about 2 years ago, and moved up to a 159 to cope with increased weight. Shorter boards are easier to turn, and easier to learn on. Longer boards are generally faster, and more stable in the chop.
A Custom X 164 will probably be quite stiff for a first board. A regular Burton Custom 160 will be less stiff, plenty long enough, and should be wide enough without needing a specific wide board. It'll still have plenty of camber and pop, being designed for people from 66-88 Kg.
Edit: If you can find a Restricted Mr Nice Guy 160, it's a little wider than the Custom, and a little less stiff. Again, should be a great first board.