What's a chunk?

I'm using the term "chunk" a lot in my notes and coding for my study.

I use the term "chunk" to refer to anything that a participant highlights or comments on as an 'unacceptable' instance of language use. I could argue that whatever they highlight with the MS word comments box thingy is the 'chunk,' but it's clear that they often highlight different things when they are actually all commenting on roughly the same few words of language.

The trouble I've been having with this is that it doesn't leave me with a very clear definition of what a "chunk" is. Is there a simple, easy-to-use taxonomy of basically all words and phrases that I can use to delineate the boundaries of chunks and describe what they are? My non-linguist gut tells me I could use SFL, but I don't know what other 'systems' / theories are in place to let me work on this easily. I guess there are some theories of 'traditional' linguistics but then I see scary things like this.

"This grammar defines the language  where  denotes a string of n consecutive 's." (From wikipedia)

My brain is broken after looking at that.

So I'm looking around for a framework that will be simple to use. On my way to check out "the Lexical Approach" by Michael Lewis, which I realize is pedagogical, but I'm just looking around.

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On another notes, for RQ1 (which is the "what do people reject" question), I am faced with sort of splitting my codes into 2 parts: one part is for the chunks themselves (which is what I'm looking for a system for, as mentioned above), and the other is what is being commented on -- that is, what exactly is being objected to, or the "linguistic concern" that the participant voices, as my supervisor put it.

The latter of these two is actually much harder to pin down. I originally had some of my constructs tangled together -- in particular, I think I was combining this one -- "what is the particpant's particular 'linguistic concern' with this chunk that is causing them to reject it" -- with a larger one that will end up being RQ2 (something like "WHY do they reject what they reject," focusing less on specific 'linguistic concerns,' though that is obviously important, and more on ideological/belief/attitude things).

The question with both parts of RQ1 is, "what is being rejected?" how much of a lexical chain, or phrase, or whatever, is being deemed 'unacceptable' ?

And why is this causing me so much grief? I think it's because I'm afraid of being wrong.