Hi krissrock,
Post #3 here has some information about “standard” sizing (which can vary from manufacturer to manufacturer) and also has a link to the Simmons standard size chart.
The standard size for Simmons for a king size is 76" x 79.5" +/- 1/2" so if it is less than 75.5" in width or 79" in length then it would be out of spec.
It’s also important to make sure you measure the mattress in between straight edges placed along the sides of the mattress rather than along the top surface which may be drawn in by the cover or tap edging. In other words it’s the widest length and width not necessarily the top surface length and width.
As you can see in this thread … there are others in the same situation who have received a mattress that is out of spec.
Phoenix