Hello Barbara, I understand that you have been using TNR, 12 point, as a font and received a negative customer review with regards to this, therefore you would like our advice as to which font to use. If you're uploading your content in DOC format, we recommend that you use "Times New Roman" font face and a font size of 12. The Kindle conversion software will then convert it to the Kindle compatible style and size. If you specify a size greater than 12, the conversion may not work well.
From experience and from reader recommendations, we suggest that you stick with the "Times New Roman".