What size SDS drill bit for red rawl plug
5.5 or 6mm sounds about right for a red plug, however not all reds are created equal, drill bits also vary and the material you are drilling into makes a difference as well. 

If I'm drilling into a softer material like plaster (especially old plaster) I will always start with a slightly smaller bit than called for in the instructions, even 1mm less,  this is because softer materials can sometimes allow the bit to drift and you end up with a larger hole than the bit. Even with harder materials like brick I will also go slightly smaller, .5mm less, than called for just in case my bit takes out more material than expected. I drill, then I try to hammer the plug in, then I will step up a size if needs be. I'd rather have a really snug fit and trim a bit of the plug off at the wall then have one that goes in but is slightly loose.  

With SDS bits I would use the same approach, how much depends on the bits and how aggressive the hammer drill is (presuming you are using one). If it's brick or concrete you can start off with a 5mm and work your way up.