When I copy and paste text from Word to Adobe CS5, the text size is reduced to half the original siz

A point is a physical size unit, 1/72 inch, from the days of lead typesetting.

In a digital image file, based on pixels, the size therefore depends on the pixels per inch number, ppi. Pixels per inch is a way to translate from pixels to physical sizes where needed.

In Microsoft applications like Word, a ppi number of 96 is assumed. So, if the Photoshop document has a different ppi, type size on screen will also differ. Set your PS document to 96 ppi and you will see that they match.

Could you please post screenshots taken at View > 100% with the pertinent Panels (Toolbar, Layers, Character, Options Bar, …) visible? 

What is the image’s resolution (Image > Image Size)?