using nexgen to display galleries

I am trying to do the following:
display a gallery of pics on a page 3 wide and six down. The thumbnail that the end user sees should be about 400×300 or so. (can be smaller)
Then when the end user clicks on the pick I would like it to enlarge to a certain size. Doesn’t matter how big, twice the size of something.
Having said that I have it loaded and it works but I cannot seem to find the setting to have the pic enlarge when it is clicked on. I am using Nexgen Basic.