8/64 warning

Every operating system requires:

Sufficient RAM for stable operation
Sufficient Flash to accommodate firmware image

Devices with 8MB flash and/or 64MB RAM will work but they will be limited regarding installing additional packages due to low flash space and/or low RAM amount. Consider this when choosing a device to buy, or when deciding to flash OpenWrt on your device.

RAM amount

RAM for stable operation:

32 MB RAM is deprecated. You will have issues with recent OpenWrt versions.
64 MB RAM may have some issues with stability, depending on your hardware and use cases, although it is enough for basic usage.
128 MB RAM or more is recommended if software beyond basic router/AP functionality is to be used.

Flash size

Flash to accommodate the firmware:

8 MB is barely enough (it may just fit LuCI and some other applications, but not many)
16MB offers slightly more flexibility, but does not allow extensive package installations. Avoid buying new devices with only 16 MB flash
>32MB is recommended
Flash size can be extended if you use the Image Generator (Image Builder) (that requires a Linux system) or use Extroot. Experienced users may create custom builds to save firmware space, but adding many packages won't fit no matter what you do.
If you want to install at least a few additional software packages, a minimum of 16MB of flash and 128MB of RAM is recommended.

Advice

Users that do not want to build their own images with features removed to reduce file size should consider 16/64 as a minimum for any device, with at least 128 MB of RAM being preferred. Users should expect that devices with less than 16 MB of flash and/or 64 MB of RAM may be unstable in basic operation and should expect that support for the device may be dropped in future.

Growth explanation

Space is needed for the bootloader (u-boot, etc.), bootloader settings, wifi firmware/calibration, OEM settings (for ability to revert to OEM firmware), and for the jffs2/ubifs overlay (configuration and storage). Thus the available space will be less than your total flash.

The Linux kernel has grown over the years with new features, bug fixes, security improvements, etc. Also OpenWrt has enabled SSL/HTTPS by default for added security which uses SSL libraries. Core packages such as LuCI have grown to include new features, and drivers have gotten larger (wifi, eth, etc.).

Care is taken by the OpenWrt developers not to add bloat without good reason. Nevertheless by this trend, OpenWrt like most software projects, will continue to grow.

Firmware size trend

As example, the sysupgrade image sizes of WNDR3700v1, an ar71xx/ath79 device, that has been supported by Openwrt for over 12 years:

Firmware       Size
main/master:   6720 KB (snapshot without LuCI)
25.12.0-rc1:   6912 kB
24.10.5:       6528 kB
23.05.6:       6016 kB
22.03.7:       6017 KB
21.02.7:       5313 KB
19.07.9:       4097 KB
18.06.8:       3712 KB
17.01.7:       3584 KB
15.05.1:       3584 KB
14.07:         3328 KB
12.09:         2816 KB

More discussion can be found in 4/32 warning.