I tried for the first time to extend the park boundaries from the original 128x128 out to 160x160. I noticed right away that the park fence did not move out with the boundary but that was okay since I didn't want the fence anyway.

So I did a little building and terrain shaping in scenario editor without any problem but when I saved the scenario as a park I discovered right away that it would not let me build anything in the extended area, only in the original 128x128 area.

Is this a glitch in the game or am I doing something wrong?

You have to expand the buyable land in the scenario editor. It's one of the buttons on the side in the park information with box with name, times, ect... A square increaser thing shows up and then you use it. NOT the tab that shows the the dimensions, that you are encountering.

Here is a YouTube video. 2:23 is the part you are missing which I mentioned.

Thanks Nathan, you're a lifesaver!