i just went and looked- and one way to figure it out would be to make the bag-as far as it goes before the lining- then measure it and adjust your lining peices to fit. remember to take seam allowances into consideration-
when i decide to try different bag sizes i usually try a sample with muslin just to get the sizes figured out. but since it is a pieced outside- it is different than just making an 8" bag into a 7" bag.--
you may want to try some scrap fabric and adjust until you get it right= remember to write down your measurments incase you want to make them again in a few months...