When sterilising my bottling bucket, do I need to totally fill it (5gal) with sterilising solution?

It is probable that you could just pasteurise the (otherwise clean) bucket using a kettle of boiling water. Boil the kettle, pour the boiled water down the sides of the bucket working round the bucket rim. When all the boiled water from the kettle is poured in you can put the lid on the bucket and tip it back and forth while rotating. The hot steam/air will cause the bucket lid to bulge - a quick vent will reduce the pressure. In such a way after (say) 2 minutes the bucket is washed and pasteurised and suitable for transferring beer into, (but empty the bucket before using it). No steriliser is wasted and the environment is a little more undisturbed - and the beer is a bit closer to nature. Ah... P.S. I have use this pasteurisation method for years. It has never failed.

I like using a spray bottle to sanitize larger equipment. There is no need to waste your supplies by filling a bucket to the top with properly diluted sanitizing solution. While swishing it around MAY get the solution in contact with each square inch of the bucket, you will feel more comfortable using a spray bottle because you can see the liquid as you spray it. Be sure to empty your spray bottle when you are done and make new solution each time you use it.

With that being said, I would like to point out that the plastic buckets used today are very good, but I try to use all glass because it is non-porous and easier to keep clean. I know that wide mouthed carboys are pretty expensive, but it can last a much longer time and you will decrease the likelihood of contamination.

Happy brewing.

Starsan is a foaming sanitizer requiring 3 minutes of contact time.

I always have better peace of mind when I fill my fermentor completly. But I've also relied on only foam conact when short on starsan / time with no I'll effects.

When preparing a secondary I fill completely then vacate with c02, but this is mainly to make the fermentor void of oxygen.

I'm going to assume that you mean "sanitize", not "sterilize".

What is the difference between Clean, Sanitized and Sterilized?

How much you need to use would depend on what you use for sanitizing. Personally I use Star San and filling the bucket with properly dilluted Star San would be a waste in my opinion.

I always keep a spray bottle with Star San solution around and use it after cleaning my fermenters. I clean my fermenters with a brush and some PBW, rinse well, then spray them with Star San solution. I make sure that the walls, the bottom and the lid of the fermenter is completely covered. I leave the fermenter with the lid on until it's time to transfer the wort, then I throw out the Star San solution that is left at the bottom of the fermenter. Star San solution is safe to drink, but I prefer keeping the Star San content in my beer to a minimum.

I can't tell you this is the best way and I don't have any experience using other sanitizers than Star San, but I can tell you that I've done this for years and I have not had an infection yet.

I usually fill it half way and make sure to coat the upper sides with those nice StarSan bubbles. I let it sit for a few minutes and then dump it out.