Jeff, the job of a jointer is to make one plane of a piece of wood flat. So if your trying to get a straight edge you would run the edge down to the cutter, with the wider part, (face), against the fence. If on the other hand your trying to remove bowing or cupping, (which would be on the face), then you must run the board face down on the jointer. All a planer will do is run a board through and remove material from one face. If the board is bowed or twisted going into the planer, it will still be bowed or twisted coming out. It's the jointers job to get a flat surface which references against the planer bed and allows the planer to make the other surface flat and parallel to the first.
As for the sizes it really depends on what your trying to accomplish. for the average guy doing a couple hours a week it's probably fine to have the jointer the same size as the planer. For larger shops the planer is almost always bigger. There are several reasons for this. For one, you can feed multiple boards at a time through the planer, the jointer not so much….(it can be done but that's not within jointing 101). You can also run wider glued up boards through a planer. And some shops buy their stock skip planed so that they don't need a bigger jointer, but can just run the stock through the planer right off.
I'm not going to argue the idea of getting wide stock flat on a narrow jointer, I don't believe it's a safe or accurate way to work, but that's my opinion and others will disagree. I'm also not going to tell you whether or not you need one as that would be silly. One must decide for themselves what tools they need to accomplish what they desire to do. I will say the jointer is a mainstay of almost every furniture and custom cabinet shop. It's considered one of the basic machines of woodworking and I couldn't imagine trying to build anything without one….but that's just me
good luck,
JeffD