Yes...you can take any photo, without regards to resolution and print to any
size. However...a 1x1 pixel image will have only the one pixel...printed
really big. 100x100 will still be pixilated...1000x1000 will be
pixilated...but maybe that will be ok. Depends on the image and your
standards and how closely you want to examine it. Canvas is forgiving as it
ads texture to make up for the lack of details. I have done 36 inches by 48
inches...that would be just under a meter to just over a meter. It can make
a difference on how the image is up-rezed. Generally when a company is
offering to do this for the general public they are counting on people being
easily satisfied.

There are certainly places that will print any photo poster size,
whether from a negative or a digital file.  The quality of the image
will still depend on the information contained in the original neg or
data file.  They still can't make a silk purse out of insufficient
information. If, however, the neg or image file is of reasonably high
resolution, and you will not be viewing it up close, these poster prints
can be useful.

Canvas has poorer rsolution capability than does a paper made for printing photographs.
So you can get away with msaking larger images that still look good - within limits.