Translation:The distance is greater than a kilometer.
I got what you mean. My confusion is because of the another example. Actually wider would be better for that example instead of longer, isn't it? https://forum.duolingo.com/comment/23030523/Το-κρεβάτι-είναι-μακρύτερο-από-το-τραπέζι
Hm, why not use either though? The dimension we're referring to (width/length) practically depends on where we're looking at (in the example in the link) the table from, so I'd say that both are fine ;)