If you are just showing a standard browser confirm box (which I think all apex.confirm does), then no you can't.
Otherwise you'd likely have to have a custom confirm box that used the jquery dialog capabilities. I won't comment on the best way to do that since I haven't worked much with the newer versions of Apex (maybe there is a plugin).
Well unfortunately no. The confirm window is controlled by the browser and will also be different on different browsers (IE, FF, Chrome, Safari, etc...)
As I was addressing your confirm inquiry I was thinking that this may be better implemented with a modal window. So look something like this Martin Giffy D'Souza on Oracle APEX: ClariFit Dialog Plugin
Or use the jQuery Dialog (you don't need to install anything to use it), the help regions use it for example.
Maybe someone else has other better suggestions.