This question arose in conversation this evening. I went looking for an answer, and thought I’d post it here for posterity and future reference. I found a source from 2003 that says it’s about 1800 square feet.
In case you’re wondering, according to the CMHC, the average size of a Canadian house in 1945 was just over 800 square feet (where did they put the kids?). In 1975, it was 1075 square feet.