it depends on your wesite layout. but i 'd suggest you to make it no more than 200kb for better loading time
I am assuming, like psychotronic, that you are talking about filesize. In this case, size is all relative. It would depend on how fast the server you are on can transmit the image and how fast the receiving browser can download it. Not everyone has a 1.4Mbit downlink (although that is rapidly becoming the norm). The filesize should be as small as you can get it. Plus you have to factor in other on-page elements (other images, css files, script files, etc.) One important thing to note is that every element requires an Http Request from the server, so the less images, scripts, and files to call the better. But I digress. I think 200K is way too large. Personally, I would shoot for no more than 35-40K. Should get a quality header at that size. I just reviewed a site here at DP, and he had page that took 8 seconds do render over a T1 1.44 line. With a little image optmization (and cleaning house on external server Http requests), we knocked that down in under 3 seconds. His original header image and theme images totaled 360K, so you can see why squeezing every filesize down is beneficial.