The camera is awfully close in this photo, but based on the size of the license plate, the scale is ten pixels per inch. The transom is roughly 6.85ft². That's 34% of the EV1's 20ft² frontal area.
EDIT: I found a better photo, with specs for front and rear track drawn on it but mislabeled (track is measured from wheel center to wheel center). It suggests a transom area of 6.35ft², or 32% of frontal area.
For the Insight, I have this excellent image to work with:
And I find a transom of about 11.5ft², 57% of my frontal area!
Though the EV1 puts my Insight to shame, you should note that the area of the detached wake is >90% of frontal area on most cars:
