Why can't D store all UTF-8 code units in char type? (not really understanding explanation)

[thebluepandabear]

Hello (noob question),

I am reading a book about D by Ali, and he talks about the 
different char types: char, wchar, and dchar. He says that char 
stores a UTF-8 code unit, wchar stores a UTF-16 code unit, and 
dchar stores a UTF-32 code unit, this makes sense.

He then goes on to say that:

"Contrary to some other programming languages, characters in D 
may consist of
different numbers of bytes. For example, because 'Ğ' must be 
represented by at
least 2 bytes in Unicode, it doesn't fit in a variable of type 
char. On the other
hand, because dchar consists of 4 bytes, it can hold any Unicode 
character."

It's his explanation as to why this code doesn't compile even 
though Ğ is a UTF-8 code unit:

```D
char utf8 = 'Ğ';
```

But I don't really understand this? What does it mean that it 
'must be represented by at least 2 bytes'? If I do `char.sizeof` 
it's 2 bytes so I am confused why it doesn't fit, I don't think 
it was explained well in the book.

Any help would be appreciated.