Manipulate the size of an instance, not the default text size.
Ideally, this should work exactly like in pcb: No need to select the text. Just let the mouse cursor hover over some text and press the shortcut. The less mouse clicks, the better. This is potentially a frequently used feature.
That sounds like a reasonable thing to support. The only exception is your request to have it affect whatever the mouse is hovering over. Destructive actions (i.e. those that change the schematic) in gschem operate on the selection, and I think it makes sense for things to remain that way.
Manipulate the text size of selected text? Or the default text size?