My brompton has disc brakes. I believe effective wheel diameter is 419mm (349 rim + 35mm tyre), compared to 698mm for 700c + 38mm tyre. I have 160mm rotors, which for stopping power would seem to equate to a somewhat unbelievable 267mm rotor on the larger wheel (160 * 698/419).
I live in one of the flattest parts of the UK so don't stress my brakes much. I am planning to cycle around the UK, some parts of which do have hills. My total weight (me + brompton + luggage) is likely to be around 120kg.
My understanding is that on normal big wheel bikes, 160mm rotors are okay for stopping power but something larger might be better for heat dissipation on longer descents.
The highest speed I have recorded myself is around 40mph coasting down a (5-10 minute long?) descent a couple of decades ago on my touring bike, at which point I decided I should apply some (rim-)braking to avoid getting any faster. These days, I think I'd probably not want to risk going much above 30mph on the brompton.
Re heat dissipation, does a smaller wheel affect the choice of rotor size?
I believe small wheels spin faster but that the amount of energy that needs to be dissipated is unrelated to wheel size. So, I would guess 160mm is still okay on the small wheel?