For now there were two available dimension sizes in Gideros SDK: device and logical. Starting from Gideros Mobile beta 7, there is also a content size available. So what is the difference between them?

Device sizes

Returns the device dimensions in pixels ignoring orientation. Meaning on an iPhone 3GS it will return width  as 320 regardless of if you're in portrait or landscape

Usage:

application:getDeviceWidth()
application:getDeviceHeight()

Logical sizes

Ignores the device dimensions and just returns whatever you've set in the "Logical dimensions" in the project properties.

Usage:

application:getLogicalWidth()
application:getLogicalHeight() 

Content sizes

And the new one, which will probably mostly used, it takes into account the orientation of the device when returning the sizes in pixels (eg on an iPhone 3GS in portrait it will return 320 but in landscape will return 480)

Usage:

application:getContentWidth()
application:getContentHeight()