They all cater for everyones needs from low to high price.
Even try E-Bay.I notice that some on there will even fit them for you for an all in one price.
I think most of those places have sales going on at the moment so maybe grab yourself a bargain.
I would imagine 900x900 maybe about the smallest size that they do and I suppose thats alright if you are nice and slim.
But as you are short of space you have no option.
Like you I am also short of space so just have a wall shower in the bath.
Stayed in a Bed and breakfast about 10 days ago and they had a small shower which was a bit claustrophobic and not pleasant to use.
Anyhow best of luck I hope you find what you need.