This article provide the information on Disk Space requirement for Junos OS Upgrades on SRX Series Firewalls

The amount of free disk space necessary to upgrade a device with a new version of Junos OS can vary from one release to another. Check the Junos OS software version you are installing to determine the free disk space requirements.

If the amount of free disk space on a device is insufficient for installing Junos OS, you might receive a warning similar to the following messages, that the /var filesystem is low on free disk space:

WARNING: The /var filesystem is low on free disk space.

WARNING: This package requires 1075136k free, but there is only 666502k available.

To determine the amount of free disk space on the device, issue the show system storage detail command. The command output displays statistics about the amount of free disk space in the device file systems.

A sample of the show system storage detail command output is shown below:

user@host> show system storage detail

Filesystem       	1024-blocks       Used      Avail   Capacity   Mounted on
/dev/da0s2a      	300196          154410     121772        56%  /
devfs                        1               1          0       100%  /dev
/dev/md0         	409000          409000          0       100%  /junos
/cf              	300196          154410     121772        56%  /junos/cf
devfs                        1               1          0       100%  /junos/dev/
procfs                       4               4          0       100%  /proc
/dev/bo0s3e              25004              52      22952         0%  /config
/dev/bo0s3f      	350628          178450     144128        55%  /cf/var
/dev/md1         	171860           16804     141308        11%  /mfs
/cf/var/jail     	350628          178450     144128        55%  /jail/var
/cf/var/log      	350628          178450     144128        55%  /jail/var/log
devfs                        1               1          0       100%  /jail/dev
/dev/md2          	40172               4      36956         0%  /mfs/var/run/utm
/dev/md3                  1884             138       1596         8%  /jail/mfs

Pay attention to the Avail space for /cf/var partition. This is the partition that is used to store temporary packages, logs, cores, etc.

Here the available space should be greater then the Junos image size.

https://www.juniper.net/documentation/us/en/software/junos/junos-install-upgrade/topics/topic-map/prepare-sotware-install-and-upgrade.html#id-ensuring-sufficient-disk-space-for-junos-os-upgrades-on-srx-devices