The mechanical sign is currently the largest sign in the game with a size of 4x3 blocks.
How to obtain
It can be crafted with level 7 engineering.
Mechanical signs can function like regular signs, displaying up to four lines of text chosen by the player. They can also be connected to any mechanical input device, such as a switch, and will display the text associated with that input device when triggered.
The sign has an option to lock the message, which prevents the message from being updated if there is one already. Triggering a locked sign with an input with no message will reset the sign.