Nah, its dead easy.
create a variable called t and one called duration, set duration to the number of seconds you want the transition to take
-- set t to min(t+dt/duration, 1)
in your on collision event.
-- lerp(canvas_size_start, canvas_size_zoomed, t^2)
That should get you started, you may need to reset t to zero after the transition has taken place.