Print size in inches, size in pixels, and definition in DPI (or more accurately, PPI) are inter-related, you cannot change just one:
print size = size in pixels / definition
which is strictly equivalent to:
size in pixels = print size * definition
or
definition = size in pixels / print size
In Image>Print size you can change the print size and the definition, but this won't change the size in pixels, because while print size and definition are just metadata and can be changed at will, changing the size in pixels alters the image data itself in irrevocable ways.
So, if you want your images to be 7.75"x7.75" at 300PPI, they need to be 2325x2325 pixels which may require scaling them.
print size = size in pixels / definition
which is strictly equivalent to:
size in pixels = print size * definition
or
definition = size in pixels / print size
In Image>Print size you can change the print size and the definition, but this won't change the size in pixels, because while print size and definition are just metadata and can be changed at will, changing the size in pixels alters the image data itself in irrevocable ways.
So, if you want your images to be 7.75"x7.75" at 300PPI, they need to be 2325x2325 pixels which may require scaling them.