(1) the area of a rhombus is equal to one half of the product of its diagonals.
(2) The diagonals of a rhombus are perpendicular.
(3) The sides of a rhombus are all the same length.
Let the diagonals be 2x and 2y. Let the side length be z.
From (2) and (3) (using Pythagoras' theorem): .... (B)
Let x have a convenient value: x = 100, say. Then from equation (A), y = 150.
Substitute x = 100 and y = 150 into equation (B) and solve for z. Note that z will not be a whole number.
(If you want z to be a whole number, then that's a much harder problem).
There are obviously many possible different answers (an infinite number, in fact) for your question unless further information is given.