Ignite nether portals of any shape or size you like, or use crying obsidian in the portal frame. All features are configurable.

Features:

• Enter "/trigger cusNetPor" to see what the min and max portal size is currently set to, even if you don't have op perms.
• Enter "/function custom_nether_portals:config" to configure
•   a) whether non-rectangular nether portals are allowed
•   b) whether crying obsidian is allowed in nether portal frames, or
•   c) the minimum and maximum nether portal size.

FAQ:

• In vanilla, the max portal frame size is 23x23 (and the min is 4x5).
• No, the data pack unfortunately cannot support portals flat on the ground.

To uninstall, enter "/function custom_nether_portals:uninstall". This will disable Custom Nether Portals and reload your data packs. After entering that command, you're good to remove the data pack file.