What's your current nexus size? How much did you increase the nexus to? Provide more information if you want help solving your problem.

Since the error message is "nexus too small", I think you can conclude that your nexus size is not set large enough. The max you should ever really set it to is 200, so try this and see if it runs. If so, then press F11 and look at the number next to the word "nex" to see how much of the nexus is being used.

You will probably need to decrease the detail of your terrain if it alone is causing the nexus to be not large enough.