theshadowknows says:

There really are no standards on how small or big things have to be, however, there are human engineering factors to take into account when building a web site. In the past, most web sites were constructed with a size optimal for a 800×600 resolution screen (about 780 pixels wide). New monitors are factory set to 1024×768 or 1280×1024 or larger. Many web developers fail to consider what their visitors will be using and make their pages whatever size fits on their monitor. An oversize screen can be very irritating for someone to scroll back and forth on. If you make it 800×600, everyone will be able to view it comfortably. As far as product images, most sites display a thumbnail which links to a near full size screen blowup.