I guess the answer is "it depends". I do not have the stock saddle or seatpost so I can not measure for you.

This is what i got when searching: https://www.google.no/search?q=size+...NoKV6ATrpY7gDQ

If you are making a case maybe make room for some padding.

A lot of peopel use the DIMPA bag from IKEA as a dIY solution. Some cardboard or sleepingmat is recomended. Can be layered betweel two DIMPA bags. Still cheap.

Edit: Looking at the pictures of folded Bromptons it looks like the stem hinge is the highest part of the bike when seat is removed- unless you have a luggageblock. If you have it looks like that would be the highest point (and therefor volnurable). Regular seatpost would be lower than the stem hinge. Not sure how tall a extended seatpost would be. I use a telescopic seatpost for easy removal when traveling.