unrar 'version 3.71 beta1 freeware' on Linux (default on Ubuntu) has the 'list archive' and 'verbosely list archive' options which would provide the unpacked size:
Code: Select all
$ unrar l SABnzbd-0.4.0Beta5.rar
UNRAR 3.71 beta 1 freeware Copyright (c) 1993-2007 Alexander Roshal
Archive SABnzbd-0.4.0Beta5.rar
Name Size Packed Ratio Date Time Attr CRC Meth Ver
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SABnzbd-0.4.0Beta5-src.tar.gz 722079 723545 100% 12-05-08 11:00 -rw-r--r-- A5C50809 m3e 2.9
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1 722079 723545 100%
Maybe as an initial release we could just do a check for (size_of_archive + 10%) which should cover the majority of tight space situations, then maybe try to refine the feature later down the road.
Thanks!