Requires the library mod Collective.
This mod is part of The Vanilla Experience.
Smaller Nether Portals is a minimalistic mod which allows the creation of smaller nether portals, specifically the sizes 1x2, 1x3 and 2x2.
Note: There is currently a "bug" in Minecraft when teleporting from a smaller portal, which places the player underneath the portal on the other side. The mod can detect when this happens, and will teleport the player in front of the portal instead. It detects the blocks below, and if it's Netherrack, Soul Sand or an Air block, it will replace those blocks with obsidian to prevent unwanted deaths. It will also create an air gap if necessary.
The smaller portals:
If you wish to do so, you may omit the corners:
Here the mod has detected the player underground after using the smaller portal, and instead placed the player in front of it: