Looking at CellControl.cs (https://github.com/MediaPortal/MediaPortal-1/blob/bf8ced0501933d2aed786e888e367c03931b82cf/mediaportal/WindowPlugins/GUINumberPlace/CellControl.cs) font is set to 'font18'. If a skin doesn't have 'font18' defined in its fonts.xml the font will be very small because it defaults to the system font.
If you try to set another <font></font> in mynumberplace.xml in <type>cell</type> it won't get used/respected. This is what should get fixed.

h4. Steps to Reproduce
1. Start MP with Titan as skin
2. Open Sudoku
3. Start a new game
4. Numbers in cells are very small because Titan fonts.xml does not contain 'font18' so it defaults to system font.

h4. Additional Information
Proposed fix from Guzzi:
http://forum.team-mediaportal.com/threads/the-path-to-1-3-0-final.116405/page-14#post-967912