JoshyL - You are probably confused because "population" means "number of people living in a place" and so "How much is the number of....?" would be quite uncomfortable, whereas "What is the number of....?" is correct. Still: I put "How big..?" in a grey-area attempt to keep DL happy, and it accepted it.
In British English, we talk about population being great or large, not big. The English here should therefore be 'How great/large is the population?', but 'great' is not accepted here.
'How big is the population?' sounds like an enquiry about the height or fatness of the population.
Here's just one example:
"An oil spill in Bengal's unique Sundarbans mangrove forest is threatening the world's greatest population of tigers."
The question resulting from this would be: "How great is the population of tigers in the Sundarbans mangrove forest of Bengal?"
Similarly, if you said to me "The world's greatest population of tigers lives in Bengal", I might respond with the question "How great is the population?" The spoken emphasis would be on 'is'.
This is a normal British English construction. Perhaps it is losing ground over time to American constructs, as is so much of the language.
I understand that translations are not always literal. Oftentimes literal translations are simply wrong. However, if this sentence is like a figure of speech then I think "How big is the population?" Should be accepted as well. At the very least, make it possible to enter the phrase that way and then explain why it's wrong when someone gets it wrong instead of making it so obtuse.
To me there is a huge difference between asking about what a population is and the size of a population. Does german really use size when talking about something they haven't yeat learned what it is? It sounds like a big error to me, and I can not understand why it is claimed to be the same as what is the size of the population. Unless it says what is the population OF ex FRANCE, "what is a population must surely be understood as wo deri g what it is. Like perhaps an event... Imagine a little girl aski g this question. "Mom, what is the population?" Would the mother say something like 9 millions, or wxplain what it is? And would the question in german that equal that one really contain the world "groß" if they whanted to know what it is. I wo der if English is really so slughish that you useot the same way, or just asumes it in a context where of a country, city ect.
In this case, without any context, that would be the correct translation.
If you'd ask "Ah, you live in a city? What's the population?" it would be correct. I speak German for around twelve years now and I never heard anyone asking me "How the population" is.
Most people would probably ask you "How many live in [INSERT CITY NAME HERE]?"