I would recommend starting here:
Nothing under 4" barrel (actually a bit over 4" as the requirement is in cm) but basically 4".
IMHO it's important to emphasize the exact regulated length: 105mm or 4.14".
This excludes many, many older American service pistols and revolvers.
However, if you look at the specs closely, you'll notice that many currently-produced so-called 4" handguns actually have barrels 4.2"-4.3" long; gunmakers often do this so they don't have to produce a special version for Canadian export!