As Miller55 said, it's personal preference. My preference is to have the bottom of the palm of my hand line up to the bottom of the palm of the glove (so the bottom palm seam would be just under the bottom of the palm of my hand) and to have my fingers just brushing the end of the gussets of the glove. If I can't brush the ends of the gussets and the palm of the gloves is a cm below the palm of my hand then the gloves feel big and sloppy to me. If my fingers are pressing through the gussets or the end of the glove palm is a cm above the palm of my hand then the gloves feel too small.
Then there's the issue of where the glove cuffs meet your elbow pads. Fortunately, the 14" Surgeon 500 gloves I use now fit like I prefer and the cuff and my elbow pads just barely brush, but don't interfere with each other.
The Hockeymonkey glove sizing page says:
If you want to know how to measure hockey gloves precisely, there's a quick two-step process to determine your correct hockey glove size.
Start by measuring the distance between the base of your palm and the top of your middle finger.
Take that number and double it. This will give you accurate, proper hockey glove sizing. For example, if the measurement is seven inches, you should purchase size 14 hockey gloves. If you measure seven and a half inches, look for size 15 hockey gloves.
If I measure from the base of my palm to the end of my middle finger it's about 8 inches. The info above would suggest I should be using 16" gloves (which I've NEVER seen and am sure I'd be swimming in), but almost every glove I've ever used has been 14". I did try a 13.5" glove once, but then sold them as my fingers were bursting through the gussets - one slash on a finger tip confirmed that it wasn't safe to go that small. In 15" gloves my fingers are about a half a cm below the ends of the gussets in most models. That could probably work, but I like to be able to just brush the ends of the gussets with most or all my fingers.
Another glove sizing guide, http://newtohockey.com/complete-hockey-gloves-guide/, suggests:
Hockey gloves are measured in (usually whole) inches. 13, 14, and 15” are usually considered adult sizes, while anything smaller is junior or youth. This length represents the distance from the bend in your inner elbow to the base of your fingers. Although this may seem like a strange way of measuring, it is a legacy from when hockey gloves were much longer than they are today.
When I measure from the bend in my inner elbow to the base of my palms, it measures exactly 14". So that way of measuring does match up with the glove size I prefer to use.
That page goes on to say:
When trying gloves on, make note of a few things for fit. If possible, spend some time shooting and stickhandling with a stick while doing this.
You should have about ¼” of space in the fingertips, and when you curl your hand into a fist, they shouldn’t stretch out the material connecting the palm to the glove at the tips.
The breaks in the fingers should be comfortable, and should line up fairly well with your knuckles. This can vary from brand to brand, so try everything on!
The wrist break should line up with your wrist on the underside.
Note from coach Jeremy (I like mobility in my gloves, my typical test is being able to give the middle finger and a thumbs up)
You don't want your fingers going past the end of the gussets as that does pose a safety hazard. At the same time, I generally don't leave 1/4" of space at the end of the fingers, maybe a mm or two at most. The knuckle and wrist breaks do line up well for me on my 14" Surgeons and the palm of the glove aligns with the palm of my hand, but all my fingers with the exception of the pinky are right at the end of the gussets (not bursting through, but just touching the end).
Based on the sizing info I've read, the general guides would suggest I should wear 15" gloves, but I prefer 14" gloves. And I'm far from a pro, but I've seen way more 13" and 14" pro stock gloves than 15" gloves; so I don't think I'm the only one who sizes down from the general rules of thumb. While I have discovered you can go too small, I haven't had protection issues with 14" gloves.