(2) The Yangtze River is about 6,300 kilometers long;

(3) Xiaohua weighs 38kg and has a house area of 90m2.

(4) The area of stamps is about 4 square centimeters;

(5) The cover area of the math book is about 3 square decimeters.

So the answer is: tons, kilometers, kilograms, square meters, square centimeters, square decimeters.