No, she just meant the sticker might be on the down tube. The measurement is supposedly of the seat tube. But with the sloping top tubes these days it's all about imaginary lines.

Prop the bike upright and use a plumb line to measure the distance from the center of the top tube where it joins the head tube, to the floor. Record this distance.

Now, find the place on your seat tube that's the same distance from the floor as what you just measured. Mark that spot with a piece of tape, then measure from that spot to the center of the bottom bracket.

That's as close as you'll get to the manufacturer's frame size.