if you are on a mac, there is this program call xscope which gives you a little box to drag around that estimates size after browser chome, scroll bars, etc...
i use it all the time...
http://iconfactory.com/software/…
just open photoshop, take a screen shot and then crop off the top
Here's what xScope says the smallest browser viewing area looks like, on both Mac and PC, with all the browsers turned on: