A 48x24-inch desk gives you enough space for a 27-inch monitor and room to move...

https://wiki-net.win/index.php/What_Are_the_Actual_Dimensions_of_a_27-Inch_Monitor%3F

A 48x24-inch desk gives you enough space for a 27-inch monitor and room to move your mouse comfortably. Leaving about 36 inches of clearance behind your chair ensures easy movement without feeling cramped