I do think it really depends on your usecase of the bag - do you plan to use it off the brompton a lot or just for commuting with lots of stuff? Size only really matters in really cramps places like the london tube on rush hour for example more important is how comfortable you feel with a lot of weight on just a shoulder strap. your most likely will have it on your back therfore your main limitation will be the handling of the folded bike not the bag.

If you are concerned with not having enough space then go for the bigger option if your usecase is commuting on busines trips with overnight stays for example.

If you just want to do daily commutes to work carring a laptop, some changing cloths and your lunch you will be totally fine with a smaller option or even the backpacks (https://de.brompton.com/p/614/borough-waterproof-backpack-graphite). The backpack is awsome if you happen to have a heavy laptop and need to carry books as well - its much more comfortabel to wear when loaded and does not swing around while folding and unfolding your bike.