@Antanicus An average lemon seed is approximately an oblate spheroid with dimensions of around 10×5.5×5.5 mm. One seed has a volume of roughly 0.16 ml.
One million lemon seeds would fill half a bathtub.
One billion lemon seeds would take up almost as much space as two average city busses.
One trillion lemon seeds would fill the main concourse of New York Grand Central Station. (or this one)