There's no single answer to the question of class size at UC Irvine. Class sizes vary dramatically depending on the course, the department, the instructor, and the level (undergraduate vs. graduate). Some introductory lectures might have hundreds of students, while upper-division seminars or graduate courses might have only a dozen.
To get a better idea of class size for specific courses, you'd need to look at the course catalog or past course offerings. You could also contact the department directly for information about typical class sizes in their programs.