You must have at least 44MB 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 220.127.116.11, “Check available memory / low memory mode” and
Section 5.2.1, “Debian Installer Parameters”).
On s390x the lowmem levels have not been tested, so automatic detection is probably outdated and you thus probably need to pass the boot parameter if your system has little memory.
Installation on systems with less memory or disk space available may be possible but is only advised for experienced users.