NavMeshAgent's shape visualization is broken! Here's a temporary fix. Attach this to a NavMeshAgent's GameObject to get an accurate visualization.

NavMeshAgentVisualizer.cs

using UnityEngine;
using UnityEngine.AI;

[RequireComponent(typeof(NavMeshAgent))]
[ExecuteInEditMode]
public class NavMeshAgentVisualizer : MonoBehaviour
{
#if UNITY_EDITOR
    private NavMeshAgent agent;
    private Mesh cylinderMesh;

    private void OnDrawGizmosSelected()
    {
        if (agent == null)
        {
            agent = GetComponent<NavMeshAgent>();
        }
        if (cylinderMesh == null)
        {
            var obj = GameObject.CreatePrimitive(PrimitiveType.Cylinder).GetComponent<MeshFilter>();
            cylinderMesh = obj.sharedMesh;
            DestroyImmediate(obj.gameObject);
        }
        var index = -1;
        for (int i = 0; i < NavMesh.GetSettingsCount(); i++)
        {
            if (NavMesh.GetSettingsByIndex(i).agentTypeID == agent.agentTypeID)
            {
                index = i;
                break;
            }
        }
        if (index != -1)
        {
            var settings = NavMesh.GetSettingsByIndex(index);
            var scale = new Vector3(settings.agentRadius * 2f, settings.agentHeight / 2f, settings.agentRadius * 2f);
            var position = transform.position + new Vector3(0f, (settings.agentHeight / 2f) - agent.baseOffset, 0f);
            var color = Gizmos.color;
            Gizmos.color = Color.cyan;
            Gizmos.DrawWireMesh(cylinderMesh, position, Quaternion.identity, scale);
            Gizmos.color = color;
        }
    }
#endif
}