What are the minimum and maximum lengths of a Mainnet Bitcoin address? I'm getting mixed answers on this question. For example, this source says addresses vary between 27 and 34 characters in length. The Wiki says 26-35 characters. Neither source cites something that will let me independently confirm the figures.

What are the minimum and maximum lengths of a Bitcoin address on mainnet, and how are these numbers derived?
The wiki is correct! The source you linked must have assumed that the address with the smallest encoding has version_byte=00, data=20*00, checksum=94A00911
$ encodeBase58 00000000000000000000000000000000000000000094A00911
1111111111111111111114oLvT2

Which has length 27. This address is valid and has been used on the blockchain! But it is not the shortest address.
I wrote this short bash script to find the minimum length address. It found that there were a total of 266 address of length 26. For anyone who is curious, these are the 266 addresses with length 26.
 
The maximum length address has version_byte=00, data=20*FF, checksum=FA06820B:
$ encodeBase58 00FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFA06820B
1QLbz7JHiBTspS962RLKV8GndWFwi5j6Qr

Which has length 34. This has also been used to receive coins, as can be seen here.