Hibernation in computing is powering down a computer while retaining its state.

Upon hibernation, the computer saves the contents of its random access memory (RAM) to a hard disk or other non-volatile storage. Upon resumption, the computer is exactly as it was upon entering hibernation.

How much disk space can i save after disabling it?

It totally depends upon your computer RAM (random access memory).

eg:- if your RAM is 4GB, than you can obviously save 4GB of disk space in C or System Partition.

How to disable it?

In windows Xp it can be simply disabled from power options,

but in windows 7 its Different.

Steps For Windows 7

1.Open Command Prompt From "All Programs >Accessories>Command Prompt " by 

 Right Clicking On it and RUN AS ADMINISTRATOR

2.Type "powercfg -h off" without quotes. this will turn it off

to turn on again  type "powercfg -h on" to turn it on.

Congratulations now you know how to disable hibernation in Windows 7 .