Characteristic impedance of a trace
As a quick rule of thumb for 50 ohms, you want the ratio of the trace width to the height above the ground plane (probably your board thickness) to be around 1.8 with 2-oz copper. With a standard 0.063" thick PCB, that means a trace width around 120 mil. (Fat!)

Formulas for calculating this can be found in various places. If you access to an engineering library, try chapter 4 of High Speed Digital Design by Johnson and Graham. (Equation 4.90 on p. 187 is the key one.)

Here's an online calculator: http://referencedesigner.com/tutorials/si/si_06.php