I think that room is easily big enough to justify a proper setup with PJ and screen. Since you are self building you also have the luxury of making any structural changes necessary to get the best out of the space before you get to far into the build
I take it that the curved back wall for the cinema room is an architectural coincidence rather than a deliberate (cinema) design element, but it sounds ideal for a curved scope screen!
In terms of screen size, I don't think there is a hard and fast rule, can you have a screen to big?
But seriously, deciding on the screen size can be a function of many aspects of the design: Where in the room can you place the PJ, what are the throw ratios and image sizes that you can achieve with your projector of choice etc. etc. This
article might be a good place to start
You can approach it from many angles whether you decide what size you would like and then choose the kit and placement to make that happen, or if you have constraints such as budget, light control, physical placement within the room, then you can work from those selecting kit that will work within those constraints then pick a screen to go with that PJ.
If immersive sound is a big priority then you might also want to look at an AT (Audio Transparent) screen which will allow centre and maybe front speakers to be placed behind the screen., so make sure you look at the beast approach for room treatment as there might be stuff you can do during the 1st fix to improve things are reduced cost or improved effectiveness vs doing it as an afterthought or retrofit.