The secret-project can probably partition disks now. I have not tried it yet.

I generalized propellor's PartSpec DSL, which had been used for only auto-fitting disk images to chroot sizes, to also support things like partitions that use some percentage of the disk.

A sample partition table using that, that gives / and /srv each 20% of the disk, has a couple of fixed size partitions, and uses the rest for /home:

[ partition EXT2 `mountedAt` "/boot"
    `setFlag` BootFlag
    `setSize` MegaBytes 512
, partition EXT4 `mountedAt` "/"
    `useDiskSpace` (Percent 20)
, partition EXT4 `mountedAt` "/srv"
    `useDiskSpace` (Percent 20)
, partition EXT4 `mountedAt` "/home"
    `useDiskSpace` RemainingSpace
, swapPartition (MegaBytes 1024)

It would probably be good to extend that with a combinator that sets a minimum allows size, so eg / can be made no smaller than 4 GB. The implementation should make it simple enough to add such combinators.

I thought about reusing the disk image auto-fitting code, so the target's / would start at the size of the installer's /. May yet do that; it would make some sense when the target and installer are fairly similar systems.