I have a very large Quickie Shadow wheelchair bag that I bought off of eBay, as I have an old-style shadow wheelchair that I adore. I thought the bag would conveniently hang on the backrest...
Until I got this monster in. It's way too large! It has one HUGE pocket, and one smaller but still big pocket on the front. This thing would fit a 18x18 or larger wheelchair, but not my small 15x16 chair.

It's 16" wide by 19" long. Front pocket is 8x10. Has two straps that go over the backrest handles of a chair, they can be adjusted. Has a bottom strap that I'm not sure of it's use...maybe to hold down the bag somehow.
Has the words Shadow by quickie on it, but the k is missing in quickie, the i is soon to come off too.
I bought it off ebay for $12, I'm looking to get that cost back, I'll ship it for $2 and I can accept paypal!