The dimensions of a Heartland Pergola refer to the width and depth of the pergola's roof. The pergola roof will include the beams and rafters, and on a traditional style pergola, will extend past the posts of your pergola.
For an example, a 14' x 14' pergola means that the overhead roof dimensions are 14' x 14'.
To figure out post spacing you will want to look at the specific CAD drawing for the pergola size you are looking for. Noting that a typical traditional style pergola will be built with the posts inset one foot from the end of your beams. Modern pergolas will NOT have an overhang.
Tip while planning - if your patio space is 14' x 14', most homeowners will opt to go a size larger on a Traditional Style Pergola so that the overhang of the pergola roof extends past the edge of your patio, rather than stopping even with it. A 14' x 14' patio would benefit from installing a 16' x 16' pergola (or greater) for optimum sun coverage.