Which size hook and bait combination

You are pretty much spot on with the size of hook and bait you are currently using. I would use 15mm boilies and a size 8 or 6 hook. If you increase the boilie size to say 20mm then a size 4 hook would be a good match. It is really about matching the hookbait and hook sizes. Hold the two together before you tie any rigs and it should be easy enough to judge what sizes work well. You will obviously want some space between the hook and hookbait, once tied, so keep that in mind. This approach will work with any type of hookbait you are thinking about using. My go to setup is 15mm Mainline Cell boilies with a piece of plastic corn to add some colour, complete with a size 8 curved hook. You will also need to think about the size of carp you are fishing for. If you are targeting the larger carp in the lake you could increase the hookbait and hook size. Alternatively, if these tactics are not working drop down a size or two and see what happens. The more effort you put in the bigger the rewards. As you gain more experience it will become second nature and you will know exactly what to use depending on the situation.

I am a fan of smaller hooks and boilies for the everyday fishing. I usually fish a size 8 hook and a 10mm single boilie. If I am going to fish for larger carp I will increase this to a size 4 hook and and use a snowman rig (a single boilie topped with a pop up to balance the hook bait). I have found this is the best rig for when I am fishing for big carp and have caught a few decent fish with this simple approach.

Hope this helps 

Don't get too zoned in on this, there is a simple old school rule that if you want to use a big bait with a small hook then just use a long hair - and with a small bait and a big hook the hair can be extremely short

Work out a few rigs you like using and stick with them, don't read too many fancy technical articles - accept that the scene has gone a bit mad (that's putting it mildly).. what I'm saying is its easy to confuse yourself

I have two or three rigs for all situations, they are very simple hair rigs with strong soft uncoated braids in 25lb. I like medium size hooks, sixes usually, and if I use an 18mm boilie I use a long hair but with a 15 or 12mm bait, or 12mm double bait, I use a short hair. I also prefer longer hooklinks, especially for bigger fish.. Kepp it simple, read the old stuff rather than the new stuff (there are a lot of prats on the scene, beware)

Cheers