My mother was also in a wheelchair - and a home - yet thought she was a lot bigger (opposite problem) so wanted a size 16/18 although she was a 12/14.
I would try to get something in the middle. 14/16 as it helps, with dressing, to have roomier clothes. Fortunately, she insisted that all labels were removed in case they scratched her (sensitive skin, like me).
I'd buy her brushed cotton nighties from Chums, remove the labels, then put her name in (all clothes had to be named) and just hope that she liked them.