Expanding filesystem on RPi3

Just installed Lede on my Raspberry Pi 3, got it working great with an asix usb-eth adapter: one eth to lan, one eth to wan, wlan to wifi, and never the twain shall meet (as is the theory of being a firewall). Already proved my hypothesis that my netcomm modem/router/firewall was already compromised (or at least just doing a bad job of being a firewall), kernel logs showing a whole bucketload of packets getting fired at my wan interface coming from the old modem/router.

But seeing as the smallest uSD cards available these days are 16GB (I could have found an 8GB if I tried hard enough, but a 16GB was only $10 so no point in looking harder), it seems a bit of a waste to only be using 250MB worth. And I wouldn't mind turning on ridiculous-logging if I've got 16GB to play with.

fdisk shows that Ive got two partitions, 1 is boot is about 20MB, 2 is root is about 240MB.
I thought I'd try the simple way first, create a 3rd partition to mount as /var/ (or use the overlay/extroot method in the documentation).
So in fdisk I created partition 3, but it only allowed me to put in a maximum of 2MiB. uh, wtf?
Then I tried creating partition 4, and that only let me create a 4MiB partition. again, wtf?
Creating an extended instead of primary partition still hits the same limits. I tried manually upping the size in the fdisk create dialogue, only to be told 'value out of range'.

So yeah, uh, what's the deal? I'm presuming this is some weirdness to do with either uSD card and/or Raspberry Pi, and what's the best way around it?

Hi, I have the same problem and still cannot find any solution. I installed it on my Banana Pi (A20) BPi-R1. Both extend the sda2 partition and create 3rd partition did not work at all. Hope some experts here can provide us with advice. Thanks!