A MOBI e-book is essentially HTML, and in particular it does not typically include explicit font sizes. By default the body of the text will be fully justified and will be in the font size selected in the Kindle menu (so all e-books have the same user-selected font size). I have never used BD, but I assume this is the case here too. So using 12-pt on the DOC file is ok.
Once you have a PRC file from BD, you can use the Windows MobiPocket Reader to see what it will look like on the Kindle. Note that the Kindle only has one font, and it is a Sans Serif font so if you view the document in MobiPocket Reader under the "A" icon select (say) the Arial font family. Also, set the line spacing to perhaps 1.5x and margins to small.
I am not convinced that Amazon's pt sizes are correct, however this may be because I have 1.5x magnification on by default under Windows, so the mapping from Windows MobiPocket Reader font sizes to Kindle sizes may be effected by that.