_blank browser window size?

If anyone can help me…
In the site im designing theres a thumbnail pic that when clicked on launches a _blank browser window to view the pic in a larger scale. How can I control the size of the new window so that its the same size as the actual pic with no white spaces??
Help!!!

launch a javascript popup with defined parameters instead of a blank window.

this sounds like a stupid question but how would i go about doing that?