You can increase size of the hard disk of an installed OS in virtual box using below command.
VBoxManage modifyhd YOUR_HARD_DISK.vdi –resize SIZE_IN_MB
YOUR_HARD_DISK.vdi – Path to your hard disk file. You can get it from the virtual box. Open the virtual box and select the VM you would like to increase the size. Click settings and goto storage. Select the VDI file under the controller. you will see the path to your vdi on the right hand side.
SIZE_IN_MB – Specify new size of the file in MB, 1024 MB is 1 GB. for 10 GB specify 10240
VBoxManage modifyhd “C:\Users\IBM_ADMIN\VirtualBox VMs\RHEL\RHEL.vdi” –resize 10240