Hi
Grid graphs have a limited resolution. It won’t be able to represent things smaller than one node that well. I’d recommend that you make the diameter of your agent equal or smaller than the width of a node.
You might want to check out recast graphs which can represent the world with more precision and still use less memory.
For 2. Take a look at this thread: How to limit searched for path length
When using the approach in that thread the path will fail if it would have been too long. You can make it return the path to the best node instead by setting the path.calculatePartial field to true.
As for 3. A 1000x1000 grid graph uses on the order of a few hundred megabytes. I don’t have the exact numbers here unfortunately.