Try this - Right click on the Title bar - you get a menu pop-up which has "Size" in it.
Click on Size, and a cruciform cursor should appear.
Hold down the left button and drag the mouse to an edge or corner of the Save As box. Keep holding the left button down, and the cursor should lock on to an edge or corner, which should then start to resize the window.
When you let go the left button, you have to start again from the beginning.