Practically there is very little that separates fermenting 1-gallon vs. 5-gallon, especially if you are doing extract or partial mash with steeping grains. However if you are doing full boils and all grain you need to have the right sized equipment for it. I'm kinda pressed for kitchen and storage space so full boils and big all grain batches are hard for me. But otherwise most everything else in the process scales in size.
You don't really need a secondary unless you are adding fruit or dry-hopping, and not even then for sure. Some people like to secondary, others do not. It involves more racking and potential for contamination, but if done carefully there isn't any particular reason why you can't do it if it makes you happy. Some people claim it makes for clearer beers, but I find just going for a longer primary takes care of that. To each his own.