I just bough some used brewing equipment from a guy on craigslist. it came with a 6 and 5gal. glass carboys. should i use the same size carboy when fermenting or could i use the different sizes during the 1st and 2nd stages.
Use the 6 gallon for primary fermentation, and then rack to the 5 gallon if you need to do long-term conditioning or whatnot. The 6 gallon will give you extra headspace to help cut down on the amount of crap that gets blown off during fermentation, and the 5 gallon should give you very little surface area so as to limit the amount of oxidation that could take place during a long storage.