Linux based development boards are all around. Raspberry Pi is probably the leader among cheap and powerful solutions, but there are more options to choose like CHIP, OrangePi, BeagleBone or other more expensive options. Raspberry Pi Zero seems to be small but it lacks some built in functionality like Ethernet. If you look for smaller Linux based computer you could give a try to VoCore. It is practically built around Ethernet plug, but features most required parts like WiFi, USB, 20+ GPIOs.
VoCore is able to run OpenWRT Linux which is capable to perform most of Linux tasks. It is suitable for home automation projects, WiFi routing, or to be placed as node in other embedded system. VoCore features Ralink/Mediatek 360MHz RT5350 MIPS processor, 32MB SDRAM and 8MB SPI flash. So it’s lightweight board (25.4mmx25.4mm) with packed functionality and price bellow $40.