I would have to make some assumptions here, based on what information you have given: such as, this control can grow dynamically to accommodate varying lengths of data?. One way to approach this would be in the Finish method (before the call to Super::Finish() ) of the control.
If your control with one line is .25 inches (base height), then in the finish get the height of the control (me.size.height) and divide by your base:
NbrLines = SafeDivide(me.size.height, .25, 0)
So if ending height is 1.25", you would have 5 lines. (Also, I used inches, but twips is the unit of measure; 1440 per inch)