Resolution Size when exporting

A good starting point is to use 360 ppi for Epson printers and 300 ppi for Canon printers. This is based on their print head designs. However, this might not be possible, and although using a lower pixel per inch, based on print viewing distance you might not notice the lower quality.

If you are saying you are going to export from Lightroom before printing I expect that you are sending the file to an external source to print. If so I would suggest that you proceed as follows.

a. Check with the printer as to the resolution of the image they require for their printing equipment. Check if they recommend a specific embedded profile e.g. RGB, Adobe RGB, ProPhoto RGB, or if they have a specific profile that they can supply.

b. If they do not supply any specific advice, I would do the following. Resize the exported image from lightroom using the following settings, Long Edge to 6000 pixels (20"x300). See the screen capture for additional info. I would expect Lightroom Classic would do a better job than the printer driver to resize the file to the required resolution for actual printing

On a decent photo ink jet, you can easily get away with 180-200 pixels per inch. You don't absolutely have to have 300 PPI. So assuming 20 inches, if you can get say 4000 pixels, with good output sharpening (which LR can provide set to Print), you'll be just fine. 

A 16x20, (is that Inches? I take it to mean inches) inch print would require the image to be cropped in the 5:4 ratio (20 / 5 = 4  4 x 4 = 16) so if you used a PPI (that is Pixels Per Inch) of 300 your image would need to be 300 pixels x 16 = 4800 and 300 x 20 = 6000 for a grand totqal of 28 Million pixels, 28 MegaPixel or larger sensor.

If your image cropped to that 5:4 ratio does not have that many pixels available then you need to decrease the resolution to the point your image will file up that 16 x 20 inch piece of paper.

I've had very good prints at a 240 PPI setting. But that all depends on Viewing Distance.

Printing on your printer or sending out to a service?

The optimal resolution is typically between 240–360 ppi depending on the printer, ink, and substrate being used (in your case, canvas). If you're sending it out, see what the printing company recommends. If you're unable to find out, a safe number is 300 ppi.