Theoretically, every tab could have its own size - most tabs having the default size, i.e. not changing the size the browser window has at the moment, but some tabs changing the window size as workspaces already do. That would mean: If the current page does not have a windows size defined, use the current window size, if it has, use the defined size and return to the prevous windows size for any link followed (that doesn't have a defined window size).
Workspaces already use different window sizes - why shouldn't tabs do the same? There could be a defined window size for certain servers that wouldn't affect other servers: If and only if a page from a defined server is displayed, the window could resize to the desired size and go back to the default size of the workspace if any other site is diplayed.