6 feet converts to 72" however I would probably go a size up to a 74" or 75" for a 15.2 hh horse, a 72 just sounds small. You can look up how to measure your horse for blanketing though in american measurements, often on the websites selling them they will have size charts
I have never heard of a 6'0 blanket measurement, but strictly mathematically, it would convert into a 72" which is quite small for a horse the size you describe. Generally a 72 would fit young/small breed horses or larger ponies; probably in the 700-800lb range. I had a pony who wore a 72 and then I sold his older used blankets to a lady who had a yearling, so that is where I estimate the size from.
Where did you get the 6'0 measurement?
1. From center of chest to center of tail;
2. From center of chest to edge of tail;
3. From center of chest to point of buttock; or
4. From point of shoulder to center of tail.
These are the most general reference points that different blanket manufactures use to determine size.
Blankets are generally "typed" as European or Western/American style. European fit differently and run in 3in increments (ie: 75, 78, 81, etc) for this type of blanket you would measure from the front point of the shoulder to the middle of the tail or from the center of the chest to the point of the buttock depending on the manufacturer. For an American style blanket you measure from the center of the chest to either the center of the tail or the edge of the tail, again depending on the manufacturer.
Most of my horses are all roughly 15-15.3 hands and over 1100lbs, but I have blankets ranging from 76-82. A fatter horse is going to require a larger blanket than a thinner one, even if they are the same height and appear to be the same length. The belly of the fatter horse will bulk out the blanket on the sides causing him to possibly need a larger size.
When in doubt, buying larger rather than smaller will prevent nasty blanket rubs. If you go too large, the blanket may not stay on properly and can easily be ruined or cause your horse to get hung up in it. Putting a blanket on that is too small won't kill them for a day or two, but it can put severe pressure on their withers or shoulders and even cause nerve damage if left too long. Usually you just wind up with a lost patch of hair though. One that is WAY too big can slip and cause a wreck by tangling in the legs while still around the horse's neck.
Check reviews for blankets you are interested in, or better yet, post them here and ask if anyone has an opinion on fit. I have some 3 season sheets that are marked 76, but measure closer to an 80 because they are made in Asian/Indonesian countries and apparently the measurements/designs don't translate very well. I received one foal blanket sized "samll" lol.