Hi!
I'm a personal trainer and can tell you that doing bench on flat focuses the work on your central pecs. If the bench is on incline it will add more focus to the upper portion of your pecs, similarly if the bench is on a decline it will add more focus to the lower portion of the pecs.
If you're looking to increase size of the muscle aim to hit the pecs hard with a variety of exercises. If you only have a bench aim to do a couple of sets on flat, a couple on incline and a couple on decline. Make sure you're not doing too many reps, for strength and bulk you'd be looking at about 8 - 10 reps, any more than that and you'd be looking to improve endurance of the muscle. Any more than 20 reps is pointless.
To get the best effects from your workout you should be looking for each rep to take between 6-8 seconds. So 3-4 seconds pressing the weight, and the same for lowering.
Select the correct weight, ie using these 6-8 second reps, the last 2 or 3 reps should be quite tough.
Also, an important point to note is that you should have at least one rest day in between chest sessions to allow the muscle to repair itself and grow. An ideal training session for upper body strength would be to work the chest and triceps one day, then back and biceps the next.
Nutrition is also important, make sure you're taking on board enough protein to repair the muscles, and also some complex carbs for refuelling otherwise all your hard work will be for nothing.
Hope these tips help.
Diz.