A comfortable shower size is a shower that is at least 48 x 36 inches.
A shower size of 48 x 36 inches provides enough space for one person to shower comfortably and can also accommodate other features like a niche or built in seat.
For people that want more space or need other accommodations, then a larger shower size of 60 inches by 36 inches is often better and is available.
Many people prefer the larger shower sizes of 48 x 48 or 60 x 36 inches.
Although the minimum comfortable shower size is 36 x 36 inches.
For a full bath with a walk in shower you should have a bathroom size of at least 40 square feet.
This allows both a shower and a bath without any constraint.
You can also just have a walk in shower if you prefer.
Or if you have 2 bathrooms, you could have a walk in shower in one bathroom and a bathtub and shower combo or bathtub only in the other bathroom.
My bathroom has a walk in shower and then also a bathtub shower combo.