Always remember to check manufacturer's website when seeking dimension data. I have attached the Gargraves Sectional Curve chart for your convenience. Interesting to note that sometimes the wood tie outer dimension (tie edge-to-tie edge) is larger than the plastic and sometimes it is just the opposite. With respect to nesting curves, some of the curve variations only provide a 7" difference. Depending on the type of equipment you run, that might be just a little tight.
As far as reach distances, most recommend 30" or less. Longer reaches can be handled if your platform is built below waist level, but that would make access to your pop-ups very difficult. For your pop-ups, I would recommend at least 3' x 3', particularly if you choose a higher platform. That will allow you more room to turn/move and work comfortably without knocking something over close to the pop-up.
Finally, it seems that you prefer table-top construction techniques which are fine. Just remember that not every table top has to be as big as the area you are filling. Look through the Layout Planning & Design Forum here and search the web for designs that use narrower tables (some with the end "bulb" to turn a train) that make good use of space while providing easier access for maintenance.
Chuck. Thanks for the insight. Much appreciated!! And the chart is awesome!! Thanks.