Learn English – Should I use very large/big/other motivation

Per this NGram, large is far more common overall than big and huge put together, so in any context where you're not sure which word to use, treat large as the default.

Also note that parents normally introduce their children to big before large (probably because it seems phonetically simpler and more distinctive). So in later life people may see big as slightly childish/informal, where large appears more "grown-up"/formal.

I suggest avoiding huge unless you really want to emphasise abnormally large. But as @barbara says, there are many alternatives in that case (enormous, gigantic, vast etc.). It's also worth noting that in casual speech, people often use "quirky/unusual" alternatives such as humungous, ginormous, thumping. It's best to avoid those unless you're in company where you notice others doing it.

The only "rule" I can think of to help decide when it's better to choose big over large is that big becomes more likely in contexts which are more metaphorical (as opposed to "literal", when you're talking about the physical size of something).

Thus, there's nothing to choose between a big man and a large man, because that's simply the literal sense. But in...

That was big of him (he did something noble/generous).
It was a big disappointment (it was very disappointing).
He's just bigging himself up (he's trying to make himself appear more important than he really is).

... large would never be used. With OP's specific noun community, the metaphoric "stretch" isn't actually very great (large/big!), and there's really nothing to choose between large and big.

Of course, there will be plenty of exceptions to my above "rule" (as in the relatively recent BrE slang giving it large, being noisily aggressive). So just think of it as a "slight tendency".

All four are correct for your situation; they just have slightly different meanings. You can also use very with any of the four choices. Think about it: all four are adjectives, so if using one is grammatically correct, then the other three will be as well.

I would alter the sentence a little, but the way you've phrased it is not wrong.

I'd put it like this:

Wow, what a * question!

Or like this:

Wow, that's a * question!

Replace * with any of the four words; all are equally correct. The use of what or that's focus the exclamation a little bit more, making the sentence a little more versatile, but aren't strictly necessary.

How about the differences in meaning?

The standout of the four words you're asking about is long; the other three are quite close in meaning, but long is distinct from them. If you say what a long question, you're remarking on the length of the content. A long question uses a lot of words (or pictures, charts, etc.) to ask.

A question described as big, huge, or large might be long, or it might be weighty, substantial, or of great significance or importance. A big question might be long but unimportant, long and important, or important but short. Here's an example of a question which is big but not long: what is the meaning of life?

Big, large and huge are all extremely close in meaning, but have slightly different magnitudes. Big and large are roughly equal (I would rate big as slightly bigger, but this is largely a matter of opinion), and huge is significantly larger than either of the other two.

Since you want to describe the length of the content of the question, long is the unambiguous choice, but you can call it big, large or huge if you want. People will understand that you mean the question has a lot of content, though depending on that content they might think you also mean the question is important or substantial in meaning.