I don’t know about you, but given how often I reinstall Windows and other OSes (see my last week) and the internet connections we have around here, it is doubly unfortunate to keep downloading VS, which in version 2015, Community edition, takes up 3.69 GB as an ISO. And it really does contain loads of stuff, including, new in version 2015, some third-party platforms, namely Git, Apache Cordova and Xamarin.
The last 2 are for developing single-code apps for various mobile operating systems. It has to be said, though, that a lot of things are always pulled from the internet, for example Android images for the emulator. 
Now let’s look at the sizes of the downloaded files for the three available VS Express versions (I will use the default names of the folders the offline installation is downloaded into, unless you change them): Express 2015 for Windows Desktop – 1.9 GB
Express 2015 for Web – 919 MB
Express 2015 for Windows 10 – 2.66 GB
The Windows Desktop and Windows 10 editions also download the full Windows 10 SDK. You can read about what it contains at https://dev.windows.com/en-us/downloads/windows-10-sdk . In both cases the folder takes up 611 MB:
And now how to download it for an offline installation: download any Express version of VS 2015 from https://www.visualstudio.com/downloads/download-visual-studio-vs . Open a command prompt in the folder where it is saved and run its installer with the /layout parameter. You will only ever get a window that does not let you choose which components to download – all of them are always downloaded. PS: This comes straight from the VS website. The information is mainly for the curious who would like to compare the sizes of the installers. Even though a single number for each of the 3 probably won’t tell you much, someone will still appreciate it.