Lights and Extra Lights

Russell is absolutely correct.  String gages are measured in Standard American Engineering Standards (SAE for short).  I don't know why.  Its just the way it is.  Most string sets are referred to as 10's, 11's, 12's etc. This is the size of the first string (high E) in the set, in thousandths of an inch; properly written .010", .011", .012" etc.   The rest of the strings follow based on a ratio of the first or smallest diameter string.  Custom sizes that are not held to normal standards are usually refered to on the packaging. Smaller sizes are, of course, easier to play. Larger gages are more difficult to play but sound better due to the tonal advantages of thicker strings (none of this means a horse's patoot if you're a metalhead cause you're playing .008's no matter what).  If you decide to play .010 gage strings on your new Martin, you may not be happy with the sound.  Acoustic guitars need thicker gages to drive their tops and produce the proper frequencies that they were designed for.  You'll have to find a happy medium between what you are comfortable playing and what sound and tonal range you find acceptable. It's a balancing act for sure.  Eventually, you'll find what works for you. Lately, I buy my strings by the dozen, so it's important to find a string set that works for you as well as a string brand or brands that fits your economic situation.