For a manuscript to submit to an agent/publisher/publication, TNR or Courier size 12. It should also be double spaced, with chapter titles halfway down a page (if applicable) and using indents rather than tab for paragraphs. For a published novel, the publisher will decide. If you're self-publishing, best to look at successful books in your genre and see what they've done. Also check out the formatting requirements of wherever you're uploading the novel.
If you're sending off to an agent or editor, they generally prefer either Times New Roman or Courier in 12 point. If you're self publishing Times New Roman, Garamond and Bookman are probably safe bets.
Do be careful with this; make sure you follow what the agent tells you in terms of formatting. Some specifically ask for double space, some ask for 1.5, some say nothing. Make really sure you follow the specific directions of whatever agency. I had submitted to agents who specifically say they want a PDF (lord knows why) and some who say ONLY .doc and nothing else, no .docx. You have to be anal with this stuff.