The perimeter depends on the length of one of the other sides. If we call the next longest side x then the perimeter will be 2003+x+sqrt(2003^2+x^2) using Pythagoras' Theorem. So the question is incomplete, because x could be any number greater than 2003.
However, if you want to keep to whole numbers there is an answer. If the missing sides have lengths 2,006,005 (hypotenuse) and 2,006,004 the perimeter is 2,006,004+2,006,005+2,003=4,014,012.