NavMeshObstacle.size

Description

The size of the obstacle, measured in the object's local space.

The size will be scaled by the transform's scale.

using UnityEngine;
using UnityEngine.AI;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    void Start()
    {
        NavMeshObstacle obstacle = GetComponent<NavMeshObstacle>();
        Mesh mesh = GetComponent<MeshFilter>().mesh;
        obstacle.shape = NavMeshObstacleShape.Box;
        obstacle.size = mesh.bounds.size;
    }
}