NextGEN thumbnail popups only half size

Trying to use NextGEN plug-in for a thumbnail gallery. Originally, and I don’t know how I did this, clicking a thumbnail would popup the image full size; e.g., 1200px x 800px. Having tinkered with setup parameters to test the plugin, the popups are now half-size (say, 600 x 400). I’ve spent at least 2 days trying to find the offending parameter that will get me back to full-size popups. Can anyone help?