In order to install Windows 10, you'll have to create a virtual hard drive for installing it. So unless you know what you're doing and have a very good reason to change it, accept the recommendation.

You can change it if you want, but I've used VirtualBox for several years and have created many virtual machines, and I've found its recommendations to be on target. You'll be shown a recommended amount of RAM. Next you'll be prompted to select how much RAM you want to devote to your Windows 10 virtual machine. Then from the Version drop-down box, select Windows 8.1, and choose either the 32-bit or 64-bit version, depending on the ISO you downloaded. From the screen that appears, type Windows 10 in the name field. Run VirtualBox and click the New icon on the upper-left side of the machine. Write down where you downloaded the ISO file, because you'll need to know that later. Some people have reported problems with installing the 64-bit version with VirtualBox, but my installation went without a hitch. I downloaded the 64-bit version, because I installed it on a 64-bit machine. Choose your language and whether you want to download the 32-bit or 64-bit version.

When you go there, click 'Get started,' and follow the instructions and prompts until you finally get to the download page.

Get the Windows 10 Technical Preview ISO File

Virtualbox download windows 10 64 bit