I have them installed on two separate hard drives (Windows 8 on my 256GB SSD and Ubuntu on a second traditional 250GB SATA HDD.)
The base install takes around 5.9GB from what I remember so depending on your usage, anything above that should do.
Cheers. I'll give Ubuntu 40gb on one of the internal SATA HDD's. Just to be safe and give a little room for apps. Thanks for the info.