I am working on a print magazine in InDesign with a smaller than normal layout: 169mm x 239mm - which body font size would be recommended for this layout size? I have tried using 8pt for the body size but at this time it appears to be too big.
The recommended sizes for print are 10-12pt however this is dependent on the typeface being used also as the structure (cap height, x-height, etc. (if you want more information on that this is a nice starting point) varies from typeface to typeface.
For the body 11pt is typically a good size but you must remember to keep your audience at the front of your mind when designing. If your audience is particularly young or of an older generation where developmental or visual concerns may come into play you might want to consider using a larger font.
Colour, line spacing, tracking, etc. will also significantly impact the overall appearance of your text also so playing around with these may produce a desirable result.
If in doubt, play it safe and go bigger than you think you'll need (without overdoing it). Better to be sure than have it illegible.
For any print, regardless of format, the optimal legbility is around font size 11pt, with ~15pt leading and ~60 characters per line (including spaces). These are of course dependent upon your target audience and other factors (like Ryan mentions). An example would be a publication aimed at seniors: they will prefer a slightly bigger font.
Of course, als Alan mentions, the typeface you use is also a huge factor, especially its x-height. Typefaces like Open Sans and Verdana remain legible at smaller point sizes due to their large x-height. As opposed to, say, Garamond.
If 11pt, or even 9pt, looks big to you, then try to change other aspects of your lay-out to compensate: closer leading, smaller margins and gutters.
8pt is not a size I'd recommend for any longer text. It's straining on the eye to read for longer periods, and not just if your eyesight is starting to fail.
I find the best way to find out whether my printed product looks good is... print it. Have your desk printer spit out a 100% preview, even if it isn't your whole page. It helps a lot to see your print, well, printed.
You're designing for print, so judging the typography from a screen is a completely different beast. As stated above, print outs will aid your design. Personally I'd print 10-20 of the same page, and adjust the typography in each. Leading, spacing, type size, until you get one that is perfect for you. What you believe to look good on screen could be type size 12, 18 leading, and the tracking turned down with zero. When on paper it could look awful. Designing for print? print examples.
This depends on the typeface(s) you are using and the volume of text. It is basicly one of those decisions you need to make when designing a large volume document. I would start drafting with an 8/10pt or 9/11pt body text style. Do a couple of pages and print them in black and white, then decide if this works or not.
If increasing the base font size, you need to see if it works with your content, how the chapters are breaking across pages and what kind of white space you get left with.
You also need to account for the overall page count: for a large document, increasing the fonts could easily add 10-15% more pages.