It isn't the DPI but the dimensions as a 16x20 print on canvas needs a minimum of 3000x2400 pixels (150DPI) so, if your image is smaller, you will need to resize it.
Have a look at: resolution - Is there a general formula for image size vs. print size? - Photography Stack Exchange it will give you an idea of what is needed
But, roughly, you can work it out using:
[print width in inches] * [ppi] = [width in pixels] -( 16 x 150 = 2400)
[print height in inches] * [ppi] = [height in pixels] -( 20 x 150 = 3000)
[EDIT] Forgot to mention:
You need to check with your print service provider as to their minimum requirements, for example SnapFish will do 1440x1800 as a 16x20 canvas print.
For a 16x20 print the first step is to crop the image to that shape (aspect ratio) which is 4:5. For a full sized 600D image that is 3456 x 4320 pixels. In other words, you crop off 864 pixels from the long side. That gives you a real ppi of 216 which for a canvas print should be more than enough and, as said above, you can crop much more from both sides, for aesthetic reasons, all the way down to 2400 x 3000. This is 40% of the original, but if you want to crop even more (to simulate a longer lens) you will have to resize it up in the Resize function on the Export page, which might entail somewhat of a loss of quality.