Since it sounds like you already ordered, this probably won't help much.
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.
That being said, MyBoyPuck is completely right about reviews and buying larger rather than smaller. I have some 3 season sheets that are marked 76, but measure closer to an 80 because they are made overseas and apparently the measurements/design doesn't transfer very well.
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.