You need to take into consideration the total file size of all artifacts making up the page and the average connection speed of your target audience which gives you the download/page presentation time.
Are you going to piss off a large portion of your target audience by making the page as a whole too cumbersome to load within an acceptable period of time. Will people give up and go elsewhere or not stay in the site as long?
If the page loading times are already only just bearable then the graphic size needs to add as little overhead as possible. If page loading is good then larger/flasher & sparklier is possible.
Assigning an arbitrary value without some basis of reality behind it is pointless if it is going to alienate your users.
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