I nearly always work in 24 bit/16 million 800x600 as opposed to 256.
I always convert to .jpg for the web as the file size is less than half
that of a .gif
and also a less grainy image...the only time I use 256 is if I am making
an image that is going to need a transparent background and then it is
easier to make it a transparent .gif
As far as max size, that is a difficulty because some people don't have
the tool bar and location showing on their browser and get the larger
surface area whereas the majority have them showing and the max full pic
without scrolling is about 400x300. We just can't be all things to all
people...
Hope this helps a little.
"Hawk"
http://art.net/~starhawk - Hawk's Domain
"Reality is a crutch for those who can't cope with Fantasy"