The visible size of a pane can be found by simple-pane-visible-height and simple-pane-visible-width, or simple-pane-visible-size (which returns two values, width and height ). Other geometric values can be accessed using with-geometry. See Width and height hints for the meaning of visible, external and internal size.
The function convert-relative-position can be used to convert coordinates between one pane or screen to another pane or screen.
Inside a static-layout (including pinboard-layout) the function static-layout-child-position and static-layout-child-size can be used to find (and set) the coordinates of a child.
Setting coordinates of panes (other than inside a static-layout) is done by the layout mechanism which is described in Laying Out CAPI Panes. In most cases, you use geometric hints or set the scroll parameters, as described in Specifying geometry hints.
CAPI User Guide and Reference Manual (Macintosh version) - 3 Aug 2017