"La puerta es pequeña."
Translation:The door is small.
I get that 'es' is permanent but would you say "la puerta está verde" rather than "es verde" as the door is green now but it may not always be green?
This is where that 'permanent/temporary' rule breaks down, and it'll confuse you more as you go forward. "Es verde" is correct because it's a characteristic of the door, repaintable or not.
Ser vs. Estar is too complicated to explain here, I recommend doing a search online for a good explanation.
Farjo1. Thanks for comment. I think it was a comfort. I'm looking forward to more Spanish challenges - maybe ...!
Luke 13:24, “Exert yourselves vigorously to get in through the narrow door, because many, I tell you, will seek to get in but will not be able.