Standing Watch is  39" x 39".  
 
If you like to over-size your blocks and trim them so they are perfect, I would strongly suggest that you use a Bloc Loc ruler. For Standing Watch, I used the Bloc Loc Flying Geese 1" x 2" ruler and  the Bloc Loc Half-Square Triangle ruler - any size from 3 1/2" - 6 1/2" will work perfectly.