The text of the Wall of Fire spell is found on page 285 of my printing of the Fifth Edition Player's Handbook. The following is all the text states regarding how the wall is shaped:
[…] You can make the wall up to 60 feet long, 20 feet high, and 1 foot thick, or a ringed wall up to 20 feet in diameter, 20 feet high, and 1 foot thick.
I believe there are two reasonable readings of this text:
- The wall is an arbitrary shape whose length does not exceed 60 feet. As a special case, if the shape is a ring, its length is instead the circumference of a circle whose diameter is 20 feet (in other words, treat pi as 3 for the sake of simplicity).
- The wall has exactly two permitted shapes: a straight line up to 60 feet long, and a circle 20 feet in diameter.
I haven't been able to find any additional clarification from official sources indicating which reading is intended, but I have found plenty of evidence indicating that different people are using both of these interpretations in play, citing indicative but not conclusive evidence outside the spell text. What is the official intended reading for this spell?