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 lowmem=2 boot parameter (see also Section 6.3.1.1, “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.