Could a human-sized brain be able to learn all the spoken languages in the world? Is our brain size capable of that not only learning them, but fluently being able to speak them? Also, if one is fluent in a language, if they don't speak it for a while, can the lose it and end up being not fluent any longer?
This question requires us to handwave a few things. For one, we have to define what a language is, as opposed to a dialect or otherwise. Usually this isn't an issue, but when you start throwing around the phrase "all the spoken languages," details like that start to be important. For sake of argument, we'll say there's 6,500 languages in the world.
Fluency is another tricky one. Non-linguists often use the word as a level of expertise: one is not fluent until they finally achieve fluency. In reality, there is no bar. One never finishes learning a language, not even their native tongue. One simply becomes more and more fluent as they practice.
However, we can work around this one with some division. Let's give our language learner a lot of luck: let's let them live to be 100 years old. To learn 6,500 languages in 100 years, you must learn 65 languages per year, every year, from birth to death. That's one language every 5.6 days, more than one language every week!
So, no, a human brain cannot learn all the spoken languages in the world, because it is strapped to a body which will give out before it can finish.
The closest we have is Daniel Temment, who managed to learn "conversational" Icelandic in a week, and German in a similar amount of time. He's also an autistic savant with synesthesia who can recite pi to 22,514 digits.
A lower bar might be to just be able to communicate with every person in the world, but that's an answer for another question!
Could a human-sized brain be able to learn all the spoken languages in the world?
If by all languages, you mean all languages with all dialects to an expert level (or become fluent), then I don't think the human brain can be able to do it.
Why? Because, as the answer by Cort Ammon put the total number of languages to be 6500+, as a data scientist I can vouch for the fact that knowing all the dialects of all the languages fluently would definitely exceed 100 TB, which is the approximate estimate of the brain's memory storage.
Also, if one is fluent in a language, if they don't speak it for a while, can the lose it and end up being not fluent any longer?
Yes, as fluency and expertise is built up through practise which involves strengthening the connections of the neurons in the brain, it is a fact that lack of usage would weaken the connections.