Hi Dan, yes my name is John :) There isn't any set limit really. It's got more to do with the performance of the device you're running it on - it depends on the amount of RAM, the performance of your CPU and GPU etc. Larger and more detailed maps will use more resources in terms of memory and processing power. So you will need to find out what your particular computer can handle.
Also, detailed objects such as trees will be more demanding than simple objects.
Generally speaking, when flying FPV you want a high and stable framerate, especially when flying at high speed. Therefore, when designing a map you will want to keep the framerate smooth. When it starts to run choppy it's a sign that you might have put too much stuff in there.