I have /var/tmp/portage on a tmpfs with about 2GiB size. As the last upgrade from gcc 4.6.3 to gcc-4.7.3-r1 was build this size was exhausted. Please add a check at the begin of the ebuild of the gcc, to ensure that there is enough space. The maschine, where this did occur, is relativly slow, so it would help people to save time. e.g. libreoffice has such a check.
There is no good value for the size of a gcc bootstrap. It'll vary a lot depending on what frontends you're building.
