The 3rd drive can be any size as long as it's big enough for your data. You can use something like Acronis to just take an image of your drive and move it back to your primary drive. The image will not be compressed.
Personally I would back up your saved files, pictures, music ect. and do a clean install of Windows. A fresh install is always faster and does not have things like driver fragments and bad registry entries to slow it down.
I think you can do what you want with the free trial of Acronis but if not it's $50 for the retail version. WD and Seagate also offer tools to do this kind of thing.