The following data taken from Nowak's classic paper shows the difficulty of relying on size to identify fossils.
Summary upper carnassial tooth length of canids: LP4
Canis dirus: 28.7 - 35.5
Canis lupus 22.2 - 30.5
Canis latrans 17.6 - 22.8
Canis familiaris 14.4 - 22.7
Canis armbrusteri 26.6 - 29.5
Canis edwardi 24.0
Canis lepophagus 19.0 - 20.7
In some cases (the fossils) the sample is small. In others, (lupus and latrans) it is more than 1