What size mountain bike do you have? You will generally have to get the saddle the same distance from the pedals on a road bike as on a mountain bike... in the same ballpark, anyways. Measure the distance from the centre of the bottom bracket to the top of your saddle, and the horizontal distance from the seat post to the top of the frame's head tube, and compare these numbers to published geometry of bikes you are considering.

In general, a 5'4" person would fit on a 'small' size bike, but all manufacturers and bodies are different.