PELLET size really comes down to the stamp of fish you are expecting. A good place to start is with a 6mm pellet, as these are easy to prepare, can be fished with a decent-sized hook, and will catch fish of all sizes.
I’d use this size for fish upwards of a pound. Remember, though, that soft pellets will be eaten by just about every fish that swims, and are quite slow-sinking. If you start to see signs that small fish are nicking the bait, be prepared to either feed larger amounts less often to ensure it gets down, or else switch to bigger pellets.
I like 8mm pellets because they do sort out the bigger fish, but they can be a little more of a pain to prepare because it is more difficult for the water to soak right through them.
I always use the ‘minute-rule’ when preparing pellets. That is, for each millimetre of their diameter I leave them to soak in the pellet pump for one minute. With 8mm pellets an eight-minute soak is about right, but
you will find that it can take at least half-an-hour for the pellets to soak right through.
Smaller 3mm and 4mm pellets are ideal for feed, but difficult to use on a reasonable hook.
They do make a useful bait when fishing for roach, though, and I fish them just as I would hemp both as feed and on the hook. These small pellets make a good feed, as they can be carefully balled-in, giving a tight bed of bait on the bottom.