I would like to add some notes:
1. Virtual box’s default HD size is 2gb. This causes problems even if it isn’t dynamic, it runs out of space while the portage is un-compressing. Even if virtual box’s HD is dynamic. This causes the tar to output errors like “tar: portage/path/to/file cannot be found”
(a gajillion of them)

2. In fdisk it asks about cylinders as well, so here is a link from the handbook that has all the information on fdisk you should need.
http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?chap=4&part=1