How to create an 3d game object in 300 X 250 size?

You don't specify what units you want, so I assume you mean world units. Select the game object in the editor, then open the Inspector and set the scale in X to 300 and the scale in Y to 250. If you want it such that the camera views the Y axis at 250 units (i.e. the cube is fully extended top to bottom of the camera), set the camera's orthographic size to 250/2 (ortho is 1/2 vertical FOV) or 125. If your camera is perspective, then you slide the cube to a Z value that provides the correct dimension (you can also calculate the camera FOV angle = arctan((1/2 cube width) / (Z delta from camera to cube front face))