Sorting out the bigger roach? which is what Stillwater asked is a slightly different proposition. certainly bigger bait which the smaller ones cant snaffle and the bigger roach can hang off the main feeding area, sometimes downstream picking off morsels; they don't like to compete with the smaller quicker fish. Trying trotting further down will sometimes pick one off or moving down 20 yards now and then and trot from there. Piece of crust with plenty of white on it about quarter to half inch square anchored down with a shot about a couple inches from the hook can be good. just trip it along the bottom or fish it static laying on style. Sweetcorn will often sort out the bigger roach as well or any similar type bait; the bites will often dry up if you switch from maggots say to sweetcorn but if your patient, a good roach or two might come.. Both baits the smaller ones will have a go at but less often. I don't have any experience of pellets but I am sure they are good as previously mentioned. Barbelboi mentions woodlice, I have found them excellent on occasions but they tend to float; mix them into balls of groundbait and one or two on the hook worth thinking about.
Features are worth thinking about, the bigger roach may be in small groups, deeper holes, alongside rushes, under bushes, trees and eddies small or large are worth looking for. Under boats are often thought of as good places but I cant say I have noticed it but might be worth a try as well.
A 14 hook buried in the bait is enough for me but you could go a 12 or 10. I generally prefer the bait static than trotting but a lot will depend on where you are. A light link lejer is as good sometimes. Aniseed is often mentioned but I have never tried it as a flavour attraction.
September coming up is a very good month for big roach. Windy, rain, low pressures and warm days are useful. Plenty of colour in the water as well.
This might help in sorting out days when the conditions are good for big roach
https://fishingweathergb.yolasite.com/
Last but not least luck, if I could bottle it or make it into a pill you could ignore all the above. However, everyone will have there favourite ways, plenty to think about than just my take on it. Should set you up in the right direction.