Gparted.
Unfortunately, gparted does not do its magic on BSD filesystems (Apart from moving them). Most of the same things can be done with growfs, disklabel, dd and fdisk. Adding ffs/disklabel support to gparted would not be a big task, but it is just one that has not been done yet.
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Note that, as run by sysinstall, fdisk makes records for the install procedure to act on latter - You can write changes immediatley, but it is not obvious how to do it.
You probably have not pressed 'W' before 'Esc' when you left the fdisk-style display in sysinstall. Yes, i know you said you did, but this is still the most likely answer!