Secondary transfer container Question

Question about secondary containers. One does size matter? If I have a 6 gallon container can I only put 5 (or less on batch size) in without any ill effects or should I fill as much as I can.

Second question, does it have to be a carboy or can it be a 6 gallon food grade bucket, like what they sell as primary?

You shouldn't have to worry about the size of your secondary as the initial fermentation is what you really need the extra space for. Since you probably have a 5 gallon batch, a 6 gallon container will be fine without question.

You can secondary in a bucket or carboy or whatever. Generally, secondarys are just for adding in hops or a flavor to the beer to add to the taste before bottling.

If it fits, you're good to go. With a secondary, the less head space the better. It doesn't have to be a carboy, a food grade bucket would work just fine.

Since a secondary fermenting vessel isn't really for fermenting (it's more properly called a "bright tank" in breweries), I don't like the idea of a bucket as a clearing vessel because of the very wide headspace. In an ale pail during fermentation, lots of co2 is produced, so it is protective of the beer and prevents any risk of oxidation. But once fermentation stops, this protective layer dissipates. Racking the beer to a new vessel means that maybe some co2 will be released, but no new co2 is being produced.

What that means is the beer will be at risk of oxidation because of the racking as well as the wide or large headspace.

I would keep my beer in the fermenter for two weeks, and not use a clearing vessel, unless I was adding more fermentables, because I don't think anything is to be gained by racking to a clearing vessel (bright tank) at this moment.

Alot of people, myself included, are leaving beer in primary for the entire fermentation and skipping secondary altogether. Here's a thread on it and you can decide what's best for you. https://www.homebrewtalk.com/f163/secondary-not-john-palmer-jamil-zainasheff-weigh-176837/