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I am considering getting a either a 7 gallon or a 14 gallon conical fermenter and am wondering if it would be okay to ferment a standard 5 gallon batch in the 14 gallon or if that would be a bad idea. Thoughts?

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You can, but the beer profile will be different. With more head space above the fermenting beer, the CO2 will not be pushed out as efficiently. This will create a higher pressure of CO2 pushing on the beer, which will decrease yeast growth, and increase ester formation.

If you want consistency then I would not recommend brewing in the wrong size vessel. If you don't care, go for it.

For further reading http://www.scientificsocieties.org/JIB/papers/1992/1992_98_6_509.pdf

EDIT

Actually I agree with brewchez. I did some further reading and it does appear that you can ferment a 5 gallon beer in a 14 gallon conical. Where what I said really applies is to fermenters that are 15' or higher. At this point the hydrostatic pressure at the bottom of the tank begins to inhibit yeast growth. Additionally on Blichmann's website, it says at the bottom that you can brew 5 gallons of beer in a 14 gallon conical.

I am now interested to trying to brew a 5 gallon batch in my 15 gallon cylindrical-conical and comparing it to a beer fermented in a glass carboy.

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    Your interpretation of the CO2 data seems flawed to me. In order to acheive the 0,1,2 BAR of over pressure with CO2 they have to have a closed system to maintain the higher pressure. In a fermentor setup described by the original poster, the CO2 would be pushing out ambient gases (air and CO2) at the same rate it was being formed for the most part. Pressure would not increase significantly in the fermentor due to headspace alone. You need the closed sysyem to build up pressure.
    – brewchez
    Jan 22, 2010 at 18:19
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The answers above are right, but I'd like to point that using a conical so big leaves much space obtained by air. I think it would be better if you used a smaller one so that less air rests inside and there is a smaller probability of the beer getting oxidized.

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    Actually, for primary fermentation you are not so worried about oxidation. During the respiration phase, the yeast require oxygen and will take in most of it. Any left in the head space will quickly be purged by the denser CO2 produced by the yeast.
    – Tim Weber
    Jan 25, 2010 at 13:35

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