Impedance on a PCB is measured using a Time Domain Reflectometer - the length of the line has no bearing on the AC impedance, it determines whether or not you line is "transmissive" - in the case of LVDS, (can go a lot higher than 200 Mhz), you need to calculate differential impedance, there are many for sale calculators and a couple of free ones, available - a new one I just found is at hxxp://www.4thdpcb.com/ -
SiGiNT
Articles can be found at hxxp://www.ultracad.com, along with a free single-ended Impedance calculator.
SiGiNT