I'm unsure if fdisk_ask_number() is the right way to request the new size. The cfdisk has a private function that uses parses_size(), so you can specify the new size as 10GiB. I guess we need to improve the dialog for fdisk, too. Possible solutions:

• implement fdisk.c-specific dialog based on get_user_reply() and parse_size()
• don't ask for a new size, but ask for a new 'Last sector' relative to the partition start (+100M to set 100MiB partition), see, for example, https://github.com/util-linux/util-linux/blob/master/libfdisk/src/gpt.c#L2551 -- this solution is probably the most "fdisk-like", but maybe overkill in this case
• introduce a new fdisk_ask_size() as an extension to FDISK_ASKTYPE_NUMBER; just add fdisk_ask_number_enable_size() and in the dialog backed (in disk-utils/fdisk.c:ask_number()) check for fdisk_ask_number_is_size() to call parse_size().

introduce a new fdisk_ask_size()

I like this the most, thanks for the suggestion!