not really but you want it wide enough to fit on your friends lap and get tucked in a bit on the sides and long enough to cover some of the legs without being too long that it gets tangled up in the chair. The size really depends on the size of the person. Probably 30" by 40" is a good starting point. Try it out by measuring that size of fabric and trying it out on your lap. You wouldn't have to cut it just fold it to that size. I would suggest you line it with fleece or flannelette so it's soft and warm

My Mom was in a wheel chair. She much preferred knitted or crocheted lap blankets because they were so much more flexible and easier to keep on her lap or her shoulders. If you're quilting one, i suggest using minky or fleece.

I was thinking of making a rag quilt, but fleece or chenille probably wouldn't "rag" very well? Maybe I'll do a strip quilt of fleece. IDK what minky is.

Minky is similar to fleece but fuzzier and softer, and often has a texture to it. Both fleece and minky stretch A LOT and are a royal pain to sew IMO.

Donna