Size for wheelchair quilt??

[Klasien]

Next summer (so I have all the time in the world ) we are planning to go to the USA again.
We will visit or stay for a couple of days with a friend of my DH (Jan), she is in a wheelchair.
Jan told me yesterday he thought it was a good idea to make her a quilt she could keep her legs warm when she is outside, I can do that and I think it will make a nice gift to give her when we see her.
Now my question.
What size do I make it?? And what is a nice pattern for a quilt like that?
Jan will ask her what colors she likes, just got myself a new project........