It depends on the tile.
Stacked pattern is less forgiving than brick pattern when you have cuts involved.
I try to design width of fire box to width of tile when doing modern.
In this project, we looked at pencil elevations to scale and took 24" wide tiles into consideration, with no cuts.
Hope this helps.
Upload your tile photo and fireplace photo if you wish, for comment. But the only way I know to be sure of final effect is to do an elevation with all elements to scale (including TV and firebox and cabinetry, etc)
We also put furniture to scale on floor plan view to ensure traffic, conversation and viewing (TV or FP) works in the room, before proceeding to place elements or purchase furnishings.

I am unsure the exact specs of that specific unit. It is by Valor. As you know, models change from year to year, so best to check with supplier for rough-in and finished specs.