On a 10-inch screen
I did a quick look through the PV+ User's Manual, but did not find a reference to touch cell size. If I recall correctly, with the PanelView Plus, the touch area for a button is determined by the size of the button, and there may no longer be touch cells in the PanelView Plus per se.
There is a button property that allows you to create a "deadband" of sorts (measured in pixels) around the edge of each button. That can help keep a user from accidentally pressing two buttons at the same time when they are spaced close together. In this case, the non-active border makes the operator have to press closer to the center of a button for it to activate.
However, the bigger (smaller?) issue may be your customer's request. IMHO, it would be difficult to create a useable touchscreen with that many elements on a larger (17")PC monitor, let alone a 10-inch HMI screen...
Rick