To determine the absolute sizes, the relative sizes are totaled and then divided evenly across the available with or height. So let's say you have two groups stacked vertically which are docked on the left side of the Form and there is 100 pixels of height in the Client area of the Form. The top group has a Size with 200 for the height and the bottom group has a Size with 100 pixels for the height. So there is a total of 300 units for the height to be distributed across 100 pixels. So every 3 units for the height would correspond to one pixel. Therefore, the top group would be 67 pixels tall and the bottom group would be 33 (they would actually be slightly less because some space is removed for the splitter between them).

So now let's say the Form size is changed and there is 200 pixels of client area and you want the top group to only occupy 50 pixels. You can calculate what it's Size's height should be based on the bottom group's Height.

FH (Form Height) = 200
TRH (top group relative height) = ?
BRH (bottom group relative height) = 100
TAH (top group absolute height = 50
BAH (bottom group absolute height) = ?

First, we have to determine the bottom group's absolute height. Obviously, the sum of both group heights equals the total Form height:
BAH + TAH = FH
If we subtract TAH from both sides, we get
BAH = FH - TAH
BAH = 200 - 50
BAH = 150

Then, since we know the relative heights relate directly to the absolute heights, we can say
TRH / TAH = BRH / BAH
If we multiple each side by TAH, we get
TRH = TAH * (BRH / BAH)
TRH = 50 * (100 / 150)
TRH = 33

So the height you assign to the top group would be 33.