As shown in the figure, Class A paper is a square with a side length of 2, Class B paper is a square with a side length of 1, and Class C paper is a rectangle with a length and width of 2 and 1 respectively. Analysis: The area of the new square must be a complete square number, which can be determined according to the area of three pieces of paper.

Answer: Solution: Class A has 1 sheet, and Class B has 4 sheets, with a total area of 4+4=8. The complete square number greater than 8 is 9 16, 25 … and the area of c is 2, so it is impossible;

When the total area is 16, the total area of C paper is 8, so it is 4.

Therefore, at least class C paper is needed before Zhang can use them to make a new square.

So the answer is: 4.

Comments: This question mainly investigates the geometric background of the complete square formula. It is the key to correctly understand that the area of the new square is a complete square number.