I built the portal 4x5, so why won't it light. I'm playing single player with all setting on. What did I do wrong
2 Answers
Please post a screenshot so we can understand your issue better. There are a few possibilities, mentioned by Sum1.
You are not lighting the portal with flint and steel.
You are leaving a gap in the portal or have an extra misplaced block. Crying Obsidian will not work for nether portals.
The Nether is disabled in your world.
There is a flowing liquid obstructing it.
I believe in older Minecraft versions (I'm going off my Console experience), you could not make a Nether portal that strayed from the standard.
In general, we'll be able to assist better if you give us what version you are on, and show us a screenshot.
I should also add that Nether portals do not function in the "Old"(Limited sized) World setting on Bedrock Edition.
If you're on the "Infinite"/default or "Flat" world types, then this restriction doesn't apply, and Nether portals should be able to function normally.