Clarify the default logo size in docstrings

Author: seisman

pygmt/src/pygmtlogo.py

@@ -305,7 +305,8 @@ def pygmtlogo(  # noqa: PLR0913
     width
     height
         Width or height of the PyGMT logo. Since the aspect ratio is fixed, only one of
-        the two can be specified.
+        the two can be specified. If not specified, the default size of the visual logo
+        is set to 2 cm.
     box
         Draw a background box behind the logo. If set to ``True``, a simple rectangular
         box is drawn using :gmt-term:`MAP_FRAME_PEN`. To customize the box appearance,