Things to keep in mind:
- Longer slides have easier recoil springs
- "Limp wristing" may be a serious problem if granny is arthritic.
- Avoid the pocket guns.
- Nothing above 9mm???? .45 typically makes for a larger grip unless you go single stack.
- There's a trade off between the light weight of polymer and felt recoil absorbing mass of metal framed guns
Maybe a full size 9mm (standard velocity loads,not +P), I'd opt for a Glock 17 Gen4 or S&W M&P. Typically lighter trigger pulls than DA/SA guns, the polymer makes them light without giving up too much in recoil absorbing mass.
Of course there's always the 4" S&W K Frame revolver.