Every font is different, so there is no definite answer – some fonts will work fine at 6pt, but others this will be far too fine or small to read.
It is important to look at the thickness of the lines that compose each letter. The finest lines of the text should be 0.25pt line thickness or equivalent (for offset printing). For other output methods, please see the relevant article about finest detail for that product.
Please note that line thickness pt and font size pt are different (eg. 8pt Arial font does not have 8pt line thickness).
As an example, the smallest size we would recommend for Helvetica Regular would be 6pt. Small font sizes like 3-5pt may print well, but will likely be very difficult to read for most people.
We recommend you always print out your design file at 100% (no scaling) to check the font sizes are able to be read and are not too small. Please note, font sizes in the Online Creator may be different to actual size.
See: Fine detail.