Size isn't an issue in this case, but would be more helpful if you provide a floor plan so we can see door / window openings. Making sure you have enough clearance for the vanity, install some grab bars and have enough turning radius for a wheelchair.

I will provide the floor plan that we have designed . Not convinced that it is the best for wheelchair accessibility . Thank you very much for responding.

I would also be happy to comment on your plan.

12×6' is a very very narrow space. your shower is going to need to all the way in the back. 5x5 foot for a roll in shower is ada. I use a wheelchair and I have a 10 1/2×12 1/2' bathroom. honestly, it's much too narrow. I would consider modifying my layout and stealing real estate from another area.