Yep, it may be a dumb question. If you are talking about the initial fermentation (the first week or two), there is no problem using a larger carboy. During that initial phase, some people use plastic pails, some use carboys, some use varying sorts of covers. It's mostly a matter of personal preference -- the wine is giving off so much CO2 during that phase that exposure to air is not really a concern. Plus the yeast generally need some oxygen to multiply properly. Most people use some sort of covering, just to keep out dust, stray cats, etc., but it's really a matter of preference and convenience.
After that, though, you need to have the wine in a sealed container, with as little head-space (ullage) as possible. If you try to keep two gallons of wine in a 5 or 6 gallon carboy, it will spoil. Doesn't matter how well you seal the container, it won't work. For two gallons of wine, you need either a two-gallon container, or two 1- gallon jugs. (You could use smaller containers, but life is too short for that, IMHO). Any review of posts on winemaking will turn up references to issues about "topping up" -- reducing the airspace above the wine surface to a minimum. If it didn't matter, you wouldn't see all those posts about topping up with water vs wine, using marbles to raise the liquid level, etc.
As to the bentonite, campden tablets (or straight potassium metabisulfite), acid blend, nutrient, etc., well, people are all different. Some drive only Fords, some wouldn't be caught dead in a Ford. It's up to you. Bentonite is helpful in removing positively charged particles (such as proteins) that can cause a haze, especially in white wines. I believe if bentonite is added before fermentation, less is needed than if added later. Do you have to use it? No. Your wine might be perfectly clear without bentonite. Or it might not. Campden tablets (pot. meta.) discourage wild yeasts and other stray micro-organisms, giving your wine yeast a little time to get firmly established. Acid blend can be very helpful, especially with fruit wines, depending on the particular fruit. A well-made wine needs the alcohol, acid and residual sugar (if any) to be in balance with each other. Wine grapes (under the right circumstances) tend to have about the right levels of acidity and sugar. If you are starting with any other fruit, you will most likely need to add sugar, or acid, or both, to reach appropriate levels. Yeast nutrient is usually a good idea, especially with certain yeasts that are fussier about nutrients. Widely-used varieties like Premier Cuvee or EC-1118 aren't as fussy about nutrients as some other yeasts; lack of nutrients can cause yeast to produce H2S (rotten egg smell) which must be eliminated before anyone will want to drink your wine. Regarding the impact of yeast variety on wine, there are (smile) varying opinions. Some studies have shown that different yeasts have significant effects on wine over the first few years, but that the differences tend to fade with time. In general, I think it is fair to say that the yeast variety will have some impact on the final product, but less than would be the case with beer. As you are a beginner, I'd recommend using a dependable, standard wine yeast such as Premier Cuvee, EC-1118 or Pasteur Red for your first couple of batches.
While you are waiting for your books to arrive in the mail, spend some time at these sites -- they are full of good stuff:
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Happy fermenting-- Doug