As a starting point, keep in mind that most corner desks have a width ranging from 20 to 40 inches, but you can find plenty of smaller options if you're tight on space and bigger models if you don't have to worry about size (jealous!);
To avoid unpleasant surprises, make sure you measure the exact spot where you're planning on placing it;
The most common width is between 20 and 30 inches. Don't forget about the chair, though! If you buy a huge corner desk with plenty of legroom but that just about fits, you might not be left with enough room to pull it out. Measure the depth of the chair if you already have one, or allow for an extra 15-20" to see if it would fit.