Right got it working
{id: "6", colour: "blue", size: "20"}
colour: "blue"
id: "6"
size: "20"
is returned when selecting blue, so now I need to get the size to then show in the size option select like so
$('#sizes').append('<option value="'+subcatObj.size+'">'+subcatObj.size+'</option>');
So the size is there within the json response but is not getting returned to the size select



