If the problem is that everything is too small for you - have you tried to zoom in with a pinch gesture?
Hope it helps,
I don't believe there is a "full screen" feature in PS Touch, at least as far as I know. You might want to suggest it in the Ideas section of this forum. I'm currently locked out of that section for some reason?
In addition to what Ig said, you can quickly get a project to 100% view by three-finger tapping in the workspace. (You can then use two-fingers to either navigate the image or pinch zoom.)