The same amount they normally eat during the year. I personally do not each much of the main course at shulchan orech so as to save some appetite for the afikomen. All other seudos I eat my normal portion size.
Given that brisket is still a relatively fatty/high cholesterol meat, even after cooking, a 4oz to 6oz portion should be plenty for a typical adult, ex=specially since you are likely to have a number of other dishes at the seder. Also, the brisket gravy itself is typically fatty if you allow the meat to “lose” its fat into the sauce while cooking and not skim it afterwards. Clearly, the fortunate younger folks or those with great metabolism and no cardio/health concerns can eat considerably more.
Chag kosher, vasameach u’bataovon!!