On GoCentral the theme you choose dictates the maximum size so that it fits in that real estate without throwing everything out of whack. So if you're at the high end of the size slider that is as large as it can be.
There is no way to change those sizes as that would require access to the background code which is not possible. Maybe another theme has a larger logo?
As far as your donate button is that code provided by a third party? If so, they would be able to advise if the size can be changed or not. If not, a workaround is to use your own graphic in the size you like and wrap the donate link code around it.
Judith "I failed my way to success." ~Thomas Edison