The design of your fire pit will make a huge difference in the amount of heat you feel. If you have a built up surround without openings, almost all your heat will go straight up from the pit, making it less important to have a lot of space. If you don't have much (or any) surround or edging to reflect the heat away from your bench, I would keep the seating at least 4 feet away, as it may be too hot to sit close otherwise.

Our fire pit at our last house was sunken and round, about 3' in diameter. It had no edging to reflect the heat, and our seating was 3' away and we could rarely sit that close. My husband is a rather *enthusiastic* fire builder, however.

I have a fire pit that is fixed and decided on movable seating not built in. That way if there is sun, wind or a better view for such things as fireworks people can adjust to suit their needs. Skip the build in seating as it will limit your options.