David’s reference to 18 points as the minimum size equates to 18 pixels if you are coming from a web background.
On some iTV projects I have pushed the type down to 16 pixels, but be very careful about colours and contrast, and enquire about the production path to air: if the work is going to be transferred via DV tape, squeezed through an old composite link, or online-edited with high compression, then you might want to leave type as large as possible.
In some cases – such as using white text on a red background – you can add a very subtle black shadow to the type, which will help stop colour bleed and crawling effects. Even if you dislike drop-shadow effects, it will still look flat and lovely on a broadcast monitor.
Safe areas need to be taken with a pinch of salt. The default safe areas in most editing and compositing software date from years ago before the widespread use of modern, widescreen televisions.
Try extending the safe area for non-essential text in interactive projects, and consult broadcaster guidelines for their widescreen policies: many channels now broadcast in 14:9 to terrestrial boxes, and offer options to satellite and cable viewers.
The largest problem is that widescreen viewers often crop the top and bottom of the image by setting their TV to crop 4:3 to 16:9. Some cable/satellite companies remove the left and right of the image to crop 16:9 to 4:3 for non-widescreen viewers, leaving us only a tiny, safe rectangle in the centre of the image to work with.
There are also excellent documents on picture standards from the BBC.
But this is one thing I don’t understand: according to the BBC: “Additional [20 or 26 horizontal] pixels are not taken into account when calculating the aspect ratio, but without them images transferred between systems will not be the correct shape.” Can anyone confirm that this is the case for PAL images?