Higher amount of Leadership grants higher party size(+10 per skill),and Renown also gives a certain amount,depending on the amount you have currently.So if you want to have a larger party,give more to Leadership,and also battle with larger parties(you have to win!),or even participate in tournaments.Both of these increase renown.And btw,by gradually battling parties in early game,you will get a certain amount of renown,but this amount will decrease,once you have attained a particular number(for eg: Having 500 renown,and then battling parties that have about the same number of men as yours,won't give you much renown,and in some cases,not even 1 renown).
Thats why you should eventually aim for the larger party-owning lords.