I would like to display a popup screen in a size that depends on the user's screen resolution. I see that the screen resolution is contained in the GuiXT system variables V[_screensize_x] and V[_screensize_y], in pixel units. In the "WindowSize" command, I need to specify the popup window size in rows and columns. How can I calculate the row/column figures from the pixel values?

This depends on the font size that the user has set up in SAP GUI. With the standard font size, the factors are 7 for the width (columns) and 11 for the height (rows). The font size is not yet contained in a GuiXT system variable, so the only thing you can do is to assume standard font size.