Feel kind of silly asking, but when I read in the David Austin catalog that a rose is "3ft x 2ft", which is the height and which is the width? I have never known the answer to this, but I figure there is some standard notation in the gardening world.
In plant catalogs and websites, (as well as the tags on the plants) the first dimension applies to height, the second is the width.
I've yet to see a third dimension added...never even thought of it for that matter !
Think of it this way:
When you are describing a person's looks to someone, you usually start by saying, "He/she is about 5' 9" tall and slim" (or whatever the case may be.) We generally start by saying how tall or short someone is, and go from there.
Plant heights and widths descriptions follow the same line of thought.
Hope that helps you remember !