You can control the size of the tiles on the Pentominoes board with Options/Set tile size...  The number displayed is the number of pixels from left to right, and from top to bottom, of each tile on the board.  The Pentominoes game window will resize appropriately when you change the tile size.

If Options/Auto-set tile size for large boards is checked, Pentominoes will make sure the game window fits on your screen whenever you start a new game or open a saved game.  Pentominoes will only decrease the tile size if necessary; it will not increase the tile size if the window already fits on your screen.  This means that you can set a tile size you like, and Pentominoes will only change it if necessary.  The smaller tile size is only temporary; it won’t affect the default size that you set with Options/Set tile size...

The smallest tile size is 15.  It is possible that very large boards (more than about 15 rows) will not fit on a 9-inch screen, even at the smallest tile size.  There is nothing I can do about this, sorry.