Bigger is usually better, but it's not always practical. Several have said already to get as long a jointer as budget and space allow. I would agree with that. I have used everything from an old 4" bench top Delta with a very short bed, to a 16" with an 8' bed. Of course I would prefer the long bed for jointing long boards, but you normally don't do that all that often, so a compromise is certainly OK. My Powermatic has a 6' bed and it does everyting I need from it. A 4' bed will certainly get the job done with some in and outfeed supports, and is much cheaper. Unless you are a production shop, you probably can't justify a bigger jointer.