To download and install R, visit cran.r-project.org/. There you will find links for installation on Linux, Mac OS, and Windows operating systems.
I also recommend using R Studio. This software is an environment for R that makes it easier to see code, output, datasets, plots, and help files together on one screen. www.rstudio.com/products/rstudio/. It is also possible to install R Studio as a portable application.
R can be installed as a portable application. This is useful in cases where you don’t want to install R on a computer, but wish to run it from a portable drive. See portableapps.com/node/32898 or sourceforge.net/projects/rportable/. My portable installation of R with a handful of added packages is about 250 MB. The version on R Studio I have is about 400 MB. So, 1 GB of space on a usb drive is probably sufficient for the software along with additional installed packages and projects.
It is also possible to access R online, without needing to install software. One example of this is R Fiddle: www.r-fiddle.org/. R Fiddle also works with common add-on packages, though I have had it refuse to use a couple of less common ones.