diff --git a/SourceGen/WpfGui/FontPicker.xaml b/SourceGen/WpfGui/FontPicker.xaml
index 60d4f2c..3fc9100 100644
--- a/SourceGen/WpfGui/FontPicker.xaml
+++ b/SourceGen/WpfGui/FontPicker.xaml
@@ -56,15 +56,20 @@ limitations under the License.
         </ListBox>
 
         <TextBlock Text="Font size:" Margin="0,8,0,0"/>
-        <ComboBox Name="sizeComboBox" IsReadOnly="True" Margin="0,4,0,0" Width="75"
-                  HorizontalAlignment="Left">
-            <!-- these are parsed directly, so only use numeric values -->
-            <ComboBoxItem>8</ComboBoxItem>
-            <ComboBoxItem>10</ComboBoxItem>
-            <ComboBoxItem>12</ComboBoxItem>
-            <ComboBoxItem>16</ComboBoxItem>
-            <ComboBoxItem>20</ComboBoxItem>
-        </ComboBox>
+        <StackPanel Orientation="Horizontal">
+            <ComboBox Name="sizeComboBox" Margin="0,4,0,0" Width="75"
+                      HorizontalAlignment="Left" IsEditable="True">
+                <!-- these are parsed directly, so only use numeric values -->
+                <ComboBoxItem>8</ComboBoxItem>
+                <ComboBoxItem>10</ComboBoxItem>
+                <ComboBoxItem>12</ComboBoxItem>
+                <ComboBoxItem>14</ComboBoxItem>
+                <ComboBoxItem>16</ComboBoxItem>
+                <ComboBoxItem>20</ComboBoxItem>
+            </ComboBox>
+            <TextBlock Name="sizeErrMsg" Margin="8,6,0,0" Text="Must be an integer from 3 to 64."
+                       Foreground="Red" Visibility="Hidden"/>
+        </StackPanel>
 
         <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Margin="0,8,0,0">
             <Button Name="okButton" Content="OK" IsDefault="True"
diff --git a/SourceGen/WpfGui/FontPicker.xaml.cs b/SourceGen/WpfGui/FontPicker.xaml.cs
index f0376b0..b926960 100644
--- a/SourceGen/WpfGui/FontPicker.xaml.cs
+++ b/SourceGen/WpfGui/FontPicker.xaml.cs
@@ -44,7 +44,7 @@ namespace SourceGen.WpfGui {
 
             GenerateMonoFontList(initialFamily);
 
-            int selIndex = 0;
+            int selIndex = -1;
             string sizeStr = initialSize.ToString();
             for (int i = 0; i < sizeComboBox.Items.Count; i++) {
                 ComboBoxItem item = (ComboBoxItem)sizeComboBox.Items[i];
@@ -53,6 +53,10 @@ namespace SourceGen.WpfGui {
                     break;
                 }
             }
+            if (selIndex < 0) {
+                // Size is not one of the standard combo box items.
+                sizeComboBox.Text = initialSize.ToString();
+            }
             sizeComboBox.SelectedIndex = selIndex;
         }
 
@@ -114,7 +118,21 @@ namespace SourceGen.WpfGui {
 
         private void OkButton_Click(object sender, RoutedEventArgs e) {
             ComboBoxItem item = (ComboBoxItem)sizeComboBox.SelectedItem;
-            SelectedSize = int.Parse((string)item.Content);
+            if (item != null) {
+                SelectedSize = int.Parse((string)item.Content);
+            } else {
+                // Catch bad font sizes when "OK" is hit.  Not as nice as disabling the OK
+                // button on bad input, but much simpler.
+                try {
+                    SelectedSize = int.Parse(sizeComboBox.Text);
+                } catch (FormatException) {
+                    SelectedSize = -1;      // trigger next test
+                }
+                if (SelectedSize < 3 || SelectedSize > 64) {
+                    sizeErrMsg.Visibility = Visibility.Visible;
+                    return;
+                }
+            }
             DialogResult = true;
         }
     }