Which version of the Application template are you using?
It does not make sense, in a way, to modify the display dimensions of a board that comes with a fixed display type, with fixed dimensions. The new ATs (V3) are now tied to values communicated from CubeMX. You can still change the dimensions manually if you want, by modifying the .part file in the root of your application next to the main .touchgfx project file (The interface to TouchGFX Generator/CubeMX). e.g. changing to 820 width for your application will cause the designer to ask for a reload and the dimensions will now be accordingly.
"AvailableResolutions" : [
{
"Width": 820,
"Height": 480
}
],