Can I make the virtualbox disk bigger

I am looking into using the info on this page

but the disk that is created in virtualbox is only 20G.  I need it more like 50-100G.  I need to have it create that way on startup and not have to alter it after vagrant is started like I do now. 

Is there a way to do this.  I looked at all the config files and could not fins anywhere to set the disks size.