Resize boot2docker VM Disk (Docker toolbox for Windows)

Stop docker machine

C:\Program Files\Docker Toolbox>docker-machine stop
Stopping "default"...
Machine "default" was stopped.

Clone existing VMDK disk to VDI disk

C:\Program Files\Oracle\VirtualBox>vboxmanage clonehd C:\Users\nutt-\.docker\machine\machines\default\disk.vmdk C:\Users\nutt-\.docker\machine\machines\default\disk.vdi --format VDI --variant Standard
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Clone medium created in format 'VDI'. UUID: ce997152-32d1-442e-b3e2-34cdc2d74d6b

Resize VDI disk to desired size

C:\Program Files\Oracle\VirtualBox>vboxmanage modifyhd C:\Users\nutt-\.docker\machine\machines\default\disk.vdi --resize 102400
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

Download GParted partition manager or use Ubuntu bootable setup CD

Add new ISO in VirtualBox as below

GParted live

Ubuntu bootable CD

Both method have the same result. Now your docker machine reside on 100GB VDI disk.