How can I make Back to my Mac Screen Sharing go full screen?
Up to 10.5.4 included, it was possible to add, among other possibilities, a full screen item to Apple Screen Sharing application : 
.
But, from 10.5.5 they remove all these features and so, the simple trick becomes more hacky.
I don't know why Apple remove these options, but anyway, if you want them back and if you're not scared about tweaking Screen Sharing application with XCode or if you have a backup of Mac OS X 10.5.4, here's an how-to.
As far as I know, the only way to get full screen is with Apple Remote Desktop or a third party VNC client.