Contact pad size for microprobe coaxial to planar waveguide transition
I assume you're asking about RF probes (like http://www.ggb.com/40a.html)-- the link you gave is for a single-pin DC probe.  There's a few concerns as to how large you would want the contact pads to be. If you make them too small it may be hard to get your probes on them, and if you accidentally damage parts of them, there's not much space left.  If you plan to eventually wire bond to the pads, it also helps to have ones that are at least 100 x 100 microns or so.  In my devices at least, any effect from the size of the contact pads is negligible compared to other impedance mismatches.  Everyone that I know of in my department uses the 150 micron pitch GSG probes just because it's a standard size, and leaves plenty of room for wire bonding or dc probes.