Trouble disabling an option in a dropdown

I'm struggling to get hold of the Toggle script for the items in the dropdown. I want to disable so game grid sizes if the device has lower resolutions.

So I have this which gets me the dropdown, but I don't know how to get access to the items so that I can use the Toggle script interactable property.

 Dropdown gridSize;
 gridSize = GameObject.Find ("GridSizeDropdown").GetComponent <Dropdown>();

Can anyone please help me out here.

Thanks