If you drop a plumb bob from your knee, be sure that the bike is sitting on a level floor, not a sloped garage floor. The bike must be sitting level for an accurate measurement. The easiest method is to drop the line from the very front of the knee to the end of the crank arm, as most-forward position. It's nothing but a starting point that is often too far forward.

If your knee is well ahead of crank arm, with the saddle all the way back, it means that the seat tube angle is too steep and/or the seatpost has too little setback. A lot of people make the mistake of buying nonsetback seatposts. Frame size has no effect on the saddle fore/aft position, since the saddle height should be the same on any frame. Smaller frames may have steeper STAs, but that depends on the brand. You may find that most brands have a 75-76 degree STA in your size.

Posting your acutal saddle height would be helpful. That would indicate whether you have long or short legs for your height.