What's the smallest text size you can use?

The smallest text size you can use depends on a number of factors. Mostly, it depends on viewing distance. And for that we have this helpful chart below.

Do remember though that if you start changing colours and fonts then things become a little more complicated. The font size chart below is based on using black Helvetica text on a white background and assumes someone with good eyesight in good light

Viewing Distance	Minimum Text Size
0.6m  /  2ft	5pt
1m  /  3.3ft	8pt
1.5m  /  5ft	13pt
2m  /  6.5ft	16pt
3m  /  10ft	25pt
5m  /  16ft	41pt
10m  /  33ft	82pt
15m  /  50ft	125pt
50m  /  160ft	410pt
60m  /  200ft	500pt
200m  /  650ft	1640pt

See for yourself

As mentioned the above chart is based on Helvetica and black text. But if you need to see what things look like in different fonts, sizes and colours then take a look at The Print Handbook. It gives examples in a range of fonts, sizes between 2pt & 10pt, and in a range of colours.