This might be over-simplifying, but... once you decide where the woman is standing, then put her feet on that spot. Then adjust her size so that her hips are at the horizon line.
If you want the woman in the blue dress to be closer, then keep her hips at the horizon line and then make her as large as you want. In the most extreme example, she might be standing six inches from the camera, in which case she is so large that we would see her hips but probably not her head and feet.
For estimating the horizon line of the garden, I looked at lines that converge to the vanishing point. In perspective drawing, you set the horizon line and the vanishing points, similar to this image here:
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For the garden scene, I looked at the angle of the hedges, the wall and the smaller hedges (all highlighted in red). Where these lines converge—which is off the page—is the vanishing point, which sits on the horizon line. I just estimated it by eye, so I may be off a bit, but you can figure it out exactly be extending the three red lines to the point where they all meet.
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