$widget -> geometry("wxh+x+y")

width (w), height (h), yposition (x), and yposition (y) can be set as string
using above syntax. Position and size can be set independently, e.g.
"+x+y"  or "100x200".
Without a parameter the actual size and position is returned in the above
syntax.
Attention: Values are "0" before the widget has manifested itself on the
screen; use
update() or waitVisibility() if unsure.

Note: All values are in pixels.

The screensize can be requested using
$top -> screenwidth()
and
$top -> screenheight()

Thus (e.g.):
$h = $top->screenheight()  - 40;
$w = $top->screenwidth() - 40;

$top ->  geometry("${w}x$h+20+20")

# if you specify -0-0 for position, you anchor in the lower right corner