The code below allows you to scale a GameObject transform to a specific size along the x & z axis by taking into account the GameObjects renderer bounds if a renderer component is attached.

/// <summary>
/// Scales a transform to specific dimensions along the x & z axis.
/// </summary>
/// <param name="transform">
/// Reference to the transform to scale.
/// </param>
/// <param name="width">The width along the x axis that represents the target size.</param>
/// <param name="height">The height along the z axis that represents the target size.</param>
public static void ScaleTransform(Transform transform, float width, float height)
{
    // get bounds of the prefab
    var bounds = new Bounds();
    var encapsulate = false;
    if (!Utilities.Helpers.GetBoundWithChildren(transform, ref bounds, ref encapsulate))
    {
        return;
    }

    // get minimum size from the size dimensions
    var min = Mathf.Min(width, height);

    // get the maximum x or z size of the transform
    var max = Mathf.Max(bounds.size.x, bounds.size.z);

    // calculate the scale factor 
    var scaleFactor = min / max;

    // apply scaling to the transform
    transform.localScale *= scaleFactor;
}

/// <summary>
/// Gets the rendering bounds of the transform.
/// </summary>
/// <param name="transform">The game object to get the bounding box for.</param>
/// <param name="pBound">The bounding box reference that will </param>
/// <param name="encapsulate">Used to determine if the first bounding box to be 
/// calculated should be encapsulated into the <see cref="pBound"/> argument.</param>
/// <returns>Returns true if at least one bounding box was calculated.</returns>
public static bool GetBoundWithChildren(Transform transform, ref Bounds pBound, ref bool encapsulate)
{
    var didOne = false;

    // get 'this' bound
    if (transform.gameObject.renderer != null)
    {
        var bound = transform.gameObject.renderer.bounds;
        if (encapsulate)
        {
            pBound.Encapsulate(bound.min);
            pBound.Encapsulate(bound.max);
        }
        else
        {
            pBound.min = bound.min;
            pBound.max = bound.max;
            encapsulate = true;
        }

        didOne = true;
    }

    // union with bound(s) of any/all children
    foreach (Transform child in transform)
    {
        if (GetBoundWithChildren(child, ref pBound, ref encapsulate))
        {
            didOne = true;
        }
    }

    return didOne;
}