Use a scale factor for link & note icons instead of an absolute size.

src/calibre/gui2/__init__.py

@@ -485,7 +485,7 @@ def create_defs():
     defs['dark_palettes'] = {}
     defs['light_palettes'] = {}
     defs['saved_layouts'] = {}
-    defs['book_details_note_link_icon_width'] = 16
+    defs['book_details_note_link_icon_width'] = 1.0
 
     def migrate_tweak(tweak_name, pref_name):
         # If the tweak has been changed then leave the tweak in the file so

src/calibre/gui2/book_details.py

@@ -15,6 +15,7 @@ from qt.core import (
     QColor,
     QDialog,
     QEasingCurve,
+    QFontMetrics,
     QIcon,
     QKeySequence,
     QMenu,
@@ -394,12 +395,15 @@ def render_data(mi, use_roman_numbers=True, all_fields=False, pref_name='book_di
                 if db.notes_for(field, item_id):
                     return item_id
 
+    # Compute the size of the link and note icons by scaling the current font
+    # height by the option specified in Preferences / L&F / Book details.
+    icon_size = int(QFontMetrics(QApplication.font()).height() * gprefs['book_details_note_link_icon_width'])
     return mi_to_html(
         mi, field_list=field_list, use_roman_numbers=use_roman_numbers, rtl=is_rtl(),
         rating_font=rating_font(), default_author_link=default_author_link(),
         comments_heading_pos=gprefs['book_details_comments_heading_pos'], for_qt=True,
         vertical_fields=vertical_fields, show_links=show_links, item_id_if_has_note=item_id_if_has_note,
-        link_note_icon_size=gprefs['book_details_note_link_icon_width']
+        link_note_icon_size=icon_size
     )
 
 # }}}

src/calibre/gui2/preferences/look_feel.ui

@@ -1003,18 +1003,22 @@ A value of zero means calculate automatically.</string>
           </widget>
          </item>
          <item>
-          <widget class="QSpinBox" name="opt_book_details_note_link_icon_width">
+          <widget class="QDoubleSpinBox" name="opt_book_details_note_link_icon_width">
            <property name="toolTip">
-            <string>Specify the size (width and height) of the links and notes icons</string>
+            <string>Increase or decrease the size of the links and notes icons by this number. Larger
+than one increases the icon size while smaller than one decreases it.</string>
            </property>
-           <property name="suffix">
-            <string> pixels</string>
-           </property>
-           <property name="maximum">
-            <number>48</number>
+           <property name="decimals">
+            <number>1</number>
            </property>
            <property name="minimum">
-            <number>12</number>
+            <double>0.5</double>
+           </property>
+           <property name="maximum">
+            <double>4.0</double>
+           </property>
+           <property name="singleStep">
+            <double>0.1</double>
            </property>
           </widget>
          </item>