It really depends of what size (and how many) cans you need and/or want in there. If you've got a nearby garage to take out excess trash fairly frequently, or you're a single person who doesn't make much waste, a small can or two can fit in a smaller pullout. If you'd like to store several days' (or a week's worth) of trash in the pullouts, and/or have lots of people contributing to waste, you'll need a bigger one. It all depends.

My trash pull out is 18" with a drawer above and 2 cans. The cans fit regular size garbage bags...maybe 13 gallon???

You can have a drawer above if you like, or none if you want room for a taller can. What size do you mean by standard cans? 21" seems plenty large for a trash pullout.

Nancy, what I meant was that a 21 inch pullout might be too big for the type of cans we have. We use that "Tall Kitchen" bags. I have never seen a 21 inch one. Cheri, sounds like you have had a variety. This kitchen is in a beach house. We can take out trash frequently if we need to, though it is nice if you don't need to do it more than once every few days. I like knowing that we have options of sizes b/c that could help with the layout of the kitchen. Thanks for input.

We just finished constructing a 12" cab with a drawer above. The trash pullout volume is 3.4 cubic feet, or about 25 gallons. So a decent sized trash can fits. It just took a bit of shopping around to find one that fit efficiently. We are using 33 gallon trash bags. In the future, we may switch to two smaller cans (with one for recycling, which is now stacked on the floor where we have not yet installed cabs...)

Same as dcward - 18" pull-out, two cans that use "Tall Kitchen" bags, drawer above (in which I store pot holders, coffee pods, and the extra SodaStream canister)

I just sized these at Ikea. To get two of their tall cans in one pull-out, you need the 18" wide base cab.

I have 2 cans in a 15", which take tall kitchen bags. Cabinets were Ikea and cans were from Lowes. YMMV, but it's just fine for our family of 4. The trash is almost too big, now that we compost.

Kristy, do you have a drawer above your 15 inch cabinet? I saw one today with two cans in it that were tiny. But, there was a drawer above.

To get a realistic idea of what's out there:

Rev-A-Shelf Trash Pullouts: http://www.rev-a-shelf.com/rev-pages/product-listing.aspx?CategoryFilterID=75

Häfele Trash Pullouts: http://www.hafele.com/us/products/trash-cans-recycling.asp

When you look at the specs for the above cabinets, keep in mind that the "minimum cabinet opening" is not the cabinet width, it's the opening in the cabinet. So, to determine the minimum width cabinet you need:

Frameless cabinets: Minimum Opening + (Thickness of cabinet walls x 2) + 1" (room for hardware, etc.)

Framed cabinets: Minimum Opening + (Thickness of face frame x 2) + 1"

I don't, but there is room. (Except when my recycle bin is overflowing with amazon boxes) I thought about adding an internal drawer, but never go around to it.

I have both (trash & Recycling) in a 17" base cabinet, to left of sink, with drawer above.