This is made possible using the immersive portals mod for fabric 1.16.5 which adds in seamless portals and Pehkui for fabric 1.16.5 for the ability to change size when entering these portals.


Mods

Immersive Portals (Fabric) https://www.curseforge.com/minecraft/...
​Immersive Portals (Forge) https://www.curseforge.com/minecraft/...
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Pehkui (Fabric) https://www.curseforge.com/minecraft/...


Fabric Installer https://fabricmc.net
Forge Installer http://files.minecraftforge.net