How big are the Java or C# reference, including their libraries?

C# 2.0/ECMA-334 4th edition [1] is specified on 553 pages - Microsoft did not submit later versions. The underlying virtual machine, the common type system, the assembly language and so on are specified in ECMA-335 6th edition [2] on 574 pages supplemented with 161,593 lines of XML (plus 155 lines DTD) specifying the base class library.

TR/84 [3] provides tools to generate documentation in various formats from the mentioned XML file. The included output as PDF sums up to 5,868 pages.

Finally there is TR/89 [4] describing some more generic types on 67 pages.

[1] http://www.ecma-international.org/publications/standards/Ecm...

[2] http://www.ecma-international.org/publications/standards/Ecm...

[3] http://www.ecma-international.org/publications/techreports/E...

[4] http://www.ecma-international.org/publications/techreports/E...

What's the relevance to Stroustrup being concise?

That they are much bigger than C++, hence Bjarne is being concise.