What font size should I use for my text?
I did my undergrad dissertation at UCL, and checking on their site, it says this:
Your thesis must be presented in a permanent and legible form in print. We would recommend using Arial or Helvetica fonts, at a size of no less than 10.
Times New Roman works OK as well, and 10 or 12 is fine. Mesquite is totally correct - as long it is professional, appropriate to an academic publication (because in theory that's what your dissertation is for), properly laid out, and readable (remember, the person marking it has to read it, so making it easy for them is a plus), you should be OK.
Check with your dept, since most universities will have a standard format (and possibly even a template
http://www.edshare.soton.ac.uk/9240/) setting out margin spacing, font, layout, etc. That will also include referencing - Harvard or footnotes? If you know well in advance, you dont suddenly need to panic when you find out its the wrong one for the work. The good thing is that Word pretty much does everything for you now, so its a lot easier. My personal advice would be to update your bibliography as you go - I didn't when I wrote up my doctoral thesis, and I had to hurriedly go through a 4 foot pile of books!
Good luck, and back up work whenever possible!