Each mantle piece changes to suit the actual firebox, tiled face and the room in which it is to sit.
Careful measurement with attention to all details will ensure a good fit. The carpenter’s axiom “Measure twice, cut once” applies.
The actual tile face and the hearth are obvious. The depth of that title from the wall, as important but more subtle.
The finished height and width of the mantle on the wall itself, of course, important factors in the ultimate designs.