According to the search results, the size of the drill bit for red plugs can vary depending on the brand and type of plug. However, most sources suggest using a 6mm drill bit for red plugs, which is the most common size. Some sources suggest using a 5.5mm drill bit for loose blockwork. It is also recommended to use a drill bit one size smaller than the anchor if using an SDS drill. It is important to note that the size of the drill bit may be indicated on the packaging or the plug itself.