Barrier-free and curbless get used interchangeably and they are not the same claim. A level entry is a necessary condition and nowhere near a sufficient one. A 900 by 900 curbless shower has no step in it and is still unusable from a wheelchair, because there is no room to get in, no room to turn and no room for anyone to assist. How the level entry is built is a floor-structure question dealt with elsewhere; this page is about the geometry that sits on top of it.
Space is the specification. A wheelchair needs somewhere to turn, commonly taken as a circle of about 1,500mm or a T-shaped space achieving the same thing in a smaller footprint, and that space can overlap the shower floor where the shower is large enough to contribute to it. A roll-in shower generally starts around 1,500 by 900mm and becomes genuinely comfortable closer to 1,500 by 1,500. Those figures usually mean the shower is not the only thing changing, because a shower that cannot be reached is not accessible.
Approach decides everything after that. Rolling in on a shower chair and transferring onto a fixed seat are two different builds. A roll-in wants an unobstructed opening, no door, and floor space on either side. A transfer shower can be considerably smaller — around 900 by 900 — but needs its seat in a defined position, the chair parked square alongside, and grab bars arranged for that specific movement. Which applies is decided by the person who will use it, and asking is the design brief.
Controls sit outside the spray, near the entry, roughly 900 to 1,100mm above the floor, so the water can be started and the temperature settled before anybody is wet — nobody reaches through a cold spray from a chair. A handheld on a slide bar with its lower end reachable while seated does most of the actual washing. Where a screen is used at all it is a fixed panel that does not narrow the approach — a hinged door across a roll-in entry cancels out the reason for building one.