You could have solid surface custom made trim.

Schluter in the stainless finish all that you will see is a tiny almost invisible line and work really well with large format porcelain tile . Make sure whoever is doing the tile work has used schluter for this type of install it Goes on before the tile This is all you will see , notice the corner where the 2 tiles meet

A mantle that is only 72" long and 8x8 seems like a terrible proportion. That's like a railroad tie. Have you actually drawn this wall elevation on paper so you know where the tile begins and ends in relationship to the firebox and all the other elements? No one likes surprises.

Good design doesn't typically happen because you think it up. It gets drawn up & the details get worked out on paper before you swing a hammer. Also, the Schluter edge required comes in scores of colors, but some of them have to be specially ordered. If your tile is charcoal gray, you might want the matching charcoal gray Schluter edge. Think ahead!

You can purchase ready-made mantles like you see above off Amazon

So what proportion do you think is appropriate for a 6ft long tile? I want 8 inches depth regardless but wasnt sure about height and the contractor suggested 8 inches due to my 20ft ceilings.

Why is the mantle only 72 " when the wall is 20 ft?

Are you installing a TV on the wall? Mantles should be longer than the TV

An 8 inch deep mantle can be 3 inches tall if that's the correct proportion.

Maybe the mantle should be as long as the wall.

Fireplace wall is 6ft long. Mantle will be 6ft long. No tv. Ceilings are 20ft, not the fireplace wall itself

Size of fire box❓
Height for mantle❓
Ceiling is 20 ft and you’re adding batten strips to the wall?…….to make it look taller?❓❓❓❓

Adding board and batten bc I love it!