How much space does Linux take?

A typical Linux installation will need somewhere between 4GB and 8GB of disk space, and you need at least a bit of space for user files, so I generally make my root partitions at least 12GB-16GB.

How big is the Linux OS?

Size of Ubuntu OS varies from 40 MB (minimal) to 4 GB. 2. Install Ubuntu over Virtual machine within windows. With the 1st option you can use the entire 8GB RAM along with the allocated amount of harddisk space while partitioning, while you can still access windows partition too.