That is the framed wall for the shower. Do you have this yet or are you still at the planning stages? You need to take everything into consideration if planning still, framing, drywall, tile, thinset, waterproofing, plmning if necessary. No one here can really just give you that dimension without know what the wall consist of and what the requirements will be for the wall depth.

Nothing has been framed - we are working from a blank canvas, so all options are able as long we I can plan into now.

Then you need to figure out how the wall needs to be framed for your particular requirements. Not something anyone here can answer. Your contractor should know. If your doing it DIY you need to determine what you need to put in the wall and how thick everything else will need to be. That is all the critical info. What do the permit drawings show?

Are you talking the outside casing? That’s wood to match their window casings. The jamb itself is stone but that would need to be determined after tile is installed but 6” would be a good approximation for the estimate. If you want to make the casing out of stone too it usually relates to something. Again I would just say 6” for now so they can finish their bid

Thanks both, super helpful!

The marble base in the photo might be a bit more than 6", but you need to think about accessibility. A young kid or an older person might have issues stepping over anything wider than 6". The height of the threshold needs to be considered too.