When re-installing any OS you have the option in clearing partitions or clearing/formatting the HDD. I'd go for the optimal decision and allocate 20GB at least for your new install. Specs are usually on the packaging of the OS's box/you can find them on the internet.
WINDOWS 8:
Referenced from:
http://windows.microsoft.com/en-GB/windows-8/system-requirements
Windows 8 system requirements
If you want to run Windows 8 on your PC, here's what it takes:
Processor: 1 gigahertz (GHz) or faster with support for PAE, NX, and SSE2 (more info)
RAM: 1 gigabyte (GB) (32-bit) or 2 GB (64-bit)
Hard disk space: 16 GB (32-bit) or 20 GB (64-bit)
Graphics card: Microsoft DirectX 9 graphics device with WDDM driver
Additional requirements to use certain features:
To use touch, you need a tablet or a monitor that supports multitouch (more info)
To access the Windows Store and to download and run apps, you need an active Internet connection and a screen resolution of at least 1024 x 768
To snap apps, you need a screen resolution of at least 1366 x 768
Internet access (ISP fees might apply)
Secure boot requires firmware that supports UEFI v2.3.1 Errata B and has the Microsoft Windows Certification Authority in the UEFI signature database
Some games and programs might require a graphics card compatible with DirectX 10 or higher for optimal performance
Microsoft account required for some features
Watching DVDs requires separate playback software (more info)
Windows Media Center license sold separately (more info)
BitLocker To Go requires a USB flash drive (Windows 8 Pro only)
BitLocker requires either Trusted Platform Module (TPM) 1.2 or a USB flash drive (Windows 8 Pro only)
Client Hyper-V requires a 64-bit system with second level address translation (SLAT) capabilities and additional 2 GB of RAM (Windows 8 Pro only)
A TV tuner is required to play and record live TV in Windows Media Center (Windows 8 Pro Pack and Windows 8 Media Center Pack only)
Free Internet TV content varies by geography, some content might require additional fees (Windows 8 Pro Pack and Windows 8 Media Center Pack only)
To check if your PC meets these requirements, you can run the Upgrade Assistant.
WINDOWS 7:
Referenced from:
http://windows.microsoft.com/systemrequirements
Windows 7 system requirements
If you want to run Windows 7 on your PC, here's what it takes:
1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
16 GB available hard disk space (32-bit) or 20 GB (64-bit)
DirectX 9 graphics device with WDDM 1.0 or higher driver
Additional requirements to use certain features:
Internet access (fees may apply)
Depending on resolution, video playback may require additional memory and advanced graphics hardware
Some games and programs might require a graphics card compatible with DirectX 10 or higher for optimal performance
For some Windows Media Center functionality a TV tuner and additional hardware may be required
Windows Touch and Tablet PCs require specific hardware
HomeGroup requires a network and PCs running Windows 7
DVD/CD authoring requires a compatible optical drive
BitLocker requires Trusted Platform Module (TPM) 1.2
BitLocker To Go requires a USB flash drive
Windows XP Mode requires an additional 1 GB of RAM and an additional 15 GB of available hard disk space.
Music and sound require audio output
Product functionality and graphics may vary based on your system configuration. Some features may require advanced or additional hardware.
PCs with multi-core processors:
Windows 7 was designed to work with today's multi-core processors. All 32-bit versions of Windows 7 can support up to 32 processor cores, while 64‑bit versions can support up to 256 processor cores.
PCs with multiple processors (CPUs):
Commercial servers, workstations, and other high-end PCs may have more than one physical processor. Windows 7 Professional, Enterprise, and Ultimate allow for two physical processors, providing the best performance on these computers. Windows 7 Starter, Home Basic, and Home Premium will recognize only one physical processor.
