Currently only the 4x5 nether portals are supported by the plugin, even though with Minecraft 1.7.2 larger portals can be build.

That means you can build larger portals and add the sign to it, but the sign never turns blue (and therefore does not change the behaviour of the portal.

Checking for this could work by seeing if there is obsidian behind a wall sign, then, if there is a nether portal block to the left or right of that then the portal must be correct

Good call, we'll see what we can do.