Have kdialog preceeded by (or succeeded after an "&" if the context allows it, and if it's preferable for some reason) a loop that has wmctrl "grepping"/activating" the unique name of the kdialog window, followed by xdotool resizing it, and breaking the loop.
Code:
( while true ; do
sleep 0.2 # reduces CPU load if the loop never breaks for some reason
wmctrl -a "relevant and ideally unique title for kdialog window" &&
...