The three dimensions all matter
The sizes here are given as length, height and depth together rather than as a single figure, and they run from around 49 inches wide up to 74, with depths from 8 inches to over 16.
Length is the overall width across the outside of the legs. Height is to the top of the shelf. Depth is how far it projects into the room, and that is the figure people forget: a mantel standing sixteen inches proud of a wall takes furniture space a flat one does not.
The opening is what it has to suit
A mantel surrounds a firebox opening, and the relationship between the two is the whole fit.
The inside dimensions of the mantel have to clear the opening and whatever the appliance manufacturer specifies around it. Those figures belong to the fireplace or the insert rather than to the stone, and they are what sets the minimum size, so the look is being chosen inside limits that already exist.
Marble leads, with wood alongside
Marble is the largest material here, with a wood group and several pieces described as rustic, barn or farmhouse.
Those are two different traditions in one collection. A carved marble mantel is a formal object with moldings and a profiled shelf; a rough wood beam is a single member with nothing done to it beyond cutting. The stone versions sit in whites, blacks and grays, the wooden ones in browns.
Polished, unfinished and textured
Polished leads the finishes, with a group listed as unfinished and one textured.
Unfinished is worth understanding rather than avoiding: it means the piece arrives without a surface treatment, ready to be painted, stained or left. It is the normal condition for a wood mantel and for a stone one meant to be finished on site to match something already in the room.
Carved, or a single member
The two traditions in this range are built in opposite ways. A carved marble mantel is assembled from shaped parts: two legs, a header across them and a shelf on top, with moldings worked into each.
A rustic beam is one member with nothing done to it beyond cutting to length. It has no legs, sits on brackets or into the wall, and the whole of its character is the face of the wood. Those are not two styles of the same product; they are two different objects that happen to occupy the same place on a wall.
White and black do opposite things
White is the commonest color here, then black, then white with gray, with browns and beiges behind them.
A white marble mantel lightens a wall and makes the firebox opening read as a dark rectangle inside a pale frame. A black one closes the whole arrangement into a single dark mass and the opening disappears into it. Those are two completely different rooms built from the same fireplace.
What this collection does not settle
Everything to do with heat, clearance and combustible materials belongs to the appliance and to whoever is fitting it, not to the stone.
A mantel is a surround, and what may sit how close to a firebox is set by the fireplace manufacturer and by local requirements. Marble furniture covers the other stone objects and fireplace inserts the appliances themselves.