The eightman is a loose forward. That means that your primary job is to chase the ball -- make tackles, ruck, generate turnovers. That's all. In any situation, if you find yourself close to ball, you have managed to get it more or less right. As for attacking, well at the very basic level, if you aren't involved in the ruck, you should position yourself to run a crash ball -- often forming a pod with another forward or two, level with the first receiver but inside of him so that the scrum half has the option of having you run a ball relatively closely to the ruck. Another option is to line up immediately behind either the first receiver or the inside centre so that you are in a position to support them should they decide to take the ball into contact. Your job in the scrum (at least on offensive) is not so much to push, but to manage the ball at the back of the scrum, and to be ready to play the ball and break off the back of the scrum, usually with a charge to the blind (or narrow) side of the pitch. You and your scrumhalf should have a prearranged signal for this -- often just a slap on the bum telling you to pick up the ball and go. At an amateur level stature will vary a lot. Almost any reasonably sized player who can make a tackle can do the job. Mobility and agression trump size in this position. At an elite level -- well AllBlack captain Richie McCaw is your ultimate loose forward. He goes at about 6'3" and 235 lbs, which is a tad large. 220 to 230 would be about the average for an international.
pretty much what the rest of the tea does. Tackle, Carry ball, pass, yada yada yada
