VirtualBox how get higher resolution with iso ?

It's been a while since I used Vbox (successfully installed CentOS). Today I did a remastersys backup (the previous one saved my bacon when I installed on my new computer). My question: I loaded the remastersys custom iso into Vbox. Good, but is there perhaps any way to get more than 800x600 res. ? Somehow I don't think it's possible as the Guest Additions cannot be mounted because the iso image is mounted. So one would actually need to install in Vbox to get the GA goodies ? Apologies if this sounds like a really silly question

 

Found a solution to the resolution problem here: http://forums.virtualbox.org/viewtopic.php?f=3&t=11182
It works well.
Have no idea re: the Guest Additions.