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As fat as you want it to be. The only time you really need to worry about it is when you're making separates. Then, of course, you want the waistline of the top and bottom to match up with each other (and with other separate pieces that are already out there). But for a full body, you can pretty much do it however you like.
Ideally, I would say it should be about as fat as the EA fat morphs, so that your sim doesn't suddenly gain a load of weight when he changes into his pyjamas, or whatever.
What I lack in decorum, I make up for with an absence of tact.