My 15" trash pull-out holds one Rubbermaid 8 gallon wastebasket, which measures 14-1/2" x 11" x 18" (the #2806, which can be found virtually anywhere). I guess there is slight tapering, but it fits grocery store paper bags perfectly. (Though in my city, they have a fairly new policy to add 5 cents/bag to your bill if you don't bring your own bag, and need grocery store bags instead.)

As an aside, I wish I had a double-bin trash pull-out so that I could have one for trash, and a second bin for recyclables. I simply did not have the room, but if you do have the room, I recommend it. My recyclables must go directly to the garage recycle bin.

I have triple trash pull-outs under a sink in a 36" cabinet in the island. (The sink actually vents in an adjacent cabinet, to conserve space). Trash, recycling and paper. New paper bags either go on the bottom (squished a little I guess) or on the side of the existing paper bag in use. 9.75 x 15.25 x 18 at the top of the bin. This is tapered, but works. Note how DH notched the side of the bin so that it fits under the sink drain.

Check out The Container Store. I have a metal can that holds my paper sacks perfectly, no taper. It was in the scratch-n-dent corner because it was missing its lid, but since I use it to hold paper recycling it's fine. Anyway, TCS has a lot of great options.

In my case.....I purchased trash cans that paper bags fit, then had the pull out made to custom fit the cans.

Not sure why you need a trashcan to hold the paper bag? My dry recycling is located behind the regular trashcan, in a paper bag, which sits directly on the pull out. There is enough room behind it to store additional paper bags. Easy enough to move the whole bag for recycle pick up day, and open up a fresh bag.