To resize a selected frame or object, first press CommandCtrl+Tab. Now one of the handles blinks to show that it is selected. To select another handle, press CommandCtrl+Tab again. Press an arrow key to resize the object by one grid unit. To resize by one pixel, hold down OptionAlt, and then press an arrow key.
The increment by which you move an object with the keyboard is determined by the document grid. To change the properties of the document grid, choose LibreOffice - PreferencesTools - Options - LibreOffice Writer - Grid.