I need to know the minimum size of a window to be considered as a fire escape on second story?

In what country and state? Codes are not universal worldwide and this is an international site.

Although there are many variables by location, many use this formula
5.7 sq.ft free space, minimum width -20", minimum height 24", maximum sill height -44". Be prudent and check with your local building permitting Dept.

Brian is right. Check with building dept. In my area minimum with is 24'' and minimum height is 24''

Chuck