The recipe that I have says:
1/4 C Margarine , 1 package (10.5 oz) marshmallows, 6 C. Rice Crispies. So if you just weigh your big marshmallows and use the amount that will equal 10.5 oz and go for it. I usually like to add a few extra marshmallows if I have them as it will just make this a little creamier and better.