I'm setting up a document and the document has some * and footnotes. The document itself is A6 size, which is pretty small (postcard size). What font-size should I assign to the smallprint footnotes?
I need the smallest appropriate size really.
I've worked on a lot of financial documents over the years (fund fact sheets, performance updates, brochures, announcement postcards, bond-issue ads) and 95% of them had footnotes and legal disclaimers at 8 pt (with body copy at 11 pt). Occasionally legalese might get reduced to 7, or in a serious pinch 6, but we usually yelled about that.
Fonts can make a difference, however. Something like Palatino, which has even stroke widths, a higher x-height, and wide, open counter spaces (the space in the loops of the letters, like "a" and "P"), will be marginally easier to read at 7 pt than something with a lower x-height and tighter counter spaces like Adobe Garamond or Bodoni's varying strokes.
I am going to have a slightly different take on this.
My main text will usually be set in 12pt, and I might go down to 10 for footnotes, but that is not always necessary. You have other "effects" at your disposal to make the footnotes not mess up/confuse/be annoying/seem too dominant.
If your main text is set in 11, 10 or 9, I would not decrease the size for the footnotes. 8 as main text, in my opinion, is pushing it too far.
To vary sizes too much in a document is often more disturbing than using gentle grayscale to differentiate content. (When going down in sizes, never go less than 2pt.)
I use a gray horisontal line, and gray text. Here is an example of going down 2pt. Click on it to see it properly:
And here is an example of sticking to 12pt:
It is important to keep in mind that font size and legibility are loosely coupled. Font size is not a measurement of what the average person would consider to be the size of a font. Or to put it another way, “Remember that long ascenders and descenders are factored into the font size.” If you don’t know much about typography there is a fair bit of information packed in a short article.
There are other factors not so far mentioned. Will the printing be on coated or uncoated stock? Is the type black on white paper (colored text can be significantly harder to read)? Is the type reversed out of the background?
The short answer is very much, it depends on the weight and font used. For myself when designing printed material, I aim for 2 pts under body (for style) and then for space or to deemphasize, or obfuscate the text (while maintaining readability) I start at about 7pts and need good cause to go below this. If I find myself hitting 6.5-5.5 I’ll also consider abusing the type by reducing space or text width (these tools should be used with extreme caution and some contempt).
6pt should be your rock bottom.
It is important to take into consideration the size of the piece. As you have stated, it is for a small post card. So taking that into account, it will be held close and read at a much closer distance than poster or 8.5 x 11.
A majority of the information on business cards for example is typically in 6pt type. Check out this link which dissects business card design.
Also check out this Map of San Francisco
Keep in mind too, as you said this is for the small print footnotes. This is the stuff you wish you could leave have, but has to be there for one reason or another. You want attract the least amount of attention possible, while still being legible.