Microsoft held its technical preview press event today in San Francisco where they showed off the first bits of the upcoming OS. As previously said, Microsoft will be releasing a technical preview for the public to try out ahead of the retail launch.
The technical preview will be available tomorrow and the downloads will come in at around 4GB for the 64-bit variety and around 3GB for the 32-bit version.
As with any preview software, especially an OS, you should not install this on production hardware as there will likely be bugs and it could break existing applications.
The download will be part of Microsoft's 'Insider' program for Windows enthusiasts; that portal will go live tomorrow as well.
This preview is considered a 'technical preview', for the consumer side of the OS, that conversation will take place in early 2015 and will likely include another preview release as well.