For a bed that we know exists but we don't know how large it is, an English speaker would say "The bed could be large." "The bed can be large" just doesn't sound right in most contexts. (Perhaps if one was in a store, and one was offered beds that can be small, medium or large.)
We were all taught from grammar school forward; that can = has the ability to be able to. And that may = has permission to do. Could = might be able to. When I saw this Q I pictured a very large couple and my girlfriend asking me, "how do they sleep?" "They can have a big bed.." but you're right, I think 'could' is a better choice. Does DL read our comments?
I came to this discussion to see what this sentence actually means as I was amazed that Duo considered my translation correct.
As it currently stands this is not something that would ever be said in english.
For it to make sense "can" would have to be changed to "could".
I guess this is just another example of how the 2 languages differ ;}