size of char type in c#

A char is unicode in C#, therefore the number of possible characters exceeds 255. So you'll need two bytes.

Extended ASCII for example has a 255-char set, and can therefore be stored in one single byte. That's also the whole purpose of the System.Text.Encoding namespace, as different systems can have different charsets, and char sizes. C# can therefore handle one/four/etc. char bytes, but Unicode UTF-16 is default.

I'm guessing with “other programming languages” you mean C. C has actually two different char types: char and wchar_t. char may be one byte long, wchar_t not necessarily.

In C# (and .NET) for that matter, all character strings are encoded as Unicode in UTF-16. That's why a char in .NET represents a single UTF-16 code unit which may be a code point or half of a surrogate pair (not actually a character, then).

Actually C#, or more accurately the CLR's, size of char is consistent with most other managed languages. Managed languages, like Java, tend to be newer and have items like unicode support built in from the ground up. The natural extension of supporting unicode strings is to have unicode char's.

Older languages like C/C++ started in ASCII only and only later added unicode support.