You must have at least 345MB of memory and 920MB of hard disk space to perform a normal installation. Note that these are fairly minimal numbers. For more realistic figures, see Section 3.4, “Meeting Minimum Hardware Requirements”.
The installer normally automatically enables memory-saving tricks to be able to
run on such low-memory system, but on architectures that are less tested
it may miss doing so. It can however be enabled manually by appending the
lowmem=1 or even
parameter (see also Section 22.214.171.124, “Check available memory / low memory mode” and
Section 5.3.2, “Debian Installer Parameters”).
Installation on systems with less memory or disk space available may be possible but is only advised for experienced users.