u can define how much disk space will be used in squid.conf. also u can define how much of RAM will be used! (give the 1/3 of RAM space which is used by squid). but i dont think that u can tell squid not to delete all cached stuff. because squid will always delete the rarely used cached stuff first, if the cache space is full. but it will always keep the most used stuffs in cache.
Code:
cache_dir ufs /var/spool/squid 100 16 256
ufs : the cache system type
/var/spool/squid : where the cache directory is sitting
100 : cache size 100 MB
16 : the number of first directories in /var/spool/cache
256 : the number of subdirectries under each that 16 directories
dont choose very high cache size. it will make your proxy slower.
good luck