As a test engineer I've already come across a couple of cases with very small letters. According to IEC 60601-1, marking should be visible at 1m by a person with 20/20 vision, in light varying from 100-1500 lx.
Many people already know this test is very subjective, so I did some research to try and get some more objectivity. Here are the results, just for reference.
From wikipedia and other sources, the definition of 20/20 vision is the ability to differentiate lines which are 1 "minute angle" apart. For a distance of 1m, this equates to 0.29mm, or 1.45mm height for the classic E shape on an eye chart.
So, any marking less than 1.5mm height clearly fails the standard.
But there are also modifiers to this basic calculation:
If the marking is mixed capitals and smalls, then you would need to make sure that the smalls are 1.5mm (making the capitals around 2.0mm).
If the font is heavy bold or another font that makes the gaps between appear smaller (e.g. an E with the legs not evenly spaced) the size should be increase to make sure gaps are at least 0.3mm.
Finally, the marking should have good contrast and good quality printing (sharp edges), otherwise again the size should be increased to compensate. For example, if the printing resolution is limited to 0.05mm, then you would need to increase the size to around 2.0mm to make sure the gaps are 0.3mm.
All in all, it seems to me that:
<1.5mm is clear fail
1.5 - 2.5 is subjective and should be checked
>2.5mm is a clear pass
The standard mentions a range of 100-1500 lx. Obviously, the low end seems to be the worst case. The higher value of 1500 lx seems to be more important for indicator lights, as per IEC 60601-1-8 (alarm indicators).
Many people already know this test is very subjective, so I did some research to try and get some more objectivity. Here are the results, just for reference.
From wikipedia and other sources, the definition of 20/20 vision is the ability to differentiate lines which are 1 "minute angle" apart. For a distance of 1m, this equates to 0.29mm, or 1.45mm height for the classic E shape on an eye chart.
So, any marking less than 1.5mm height clearly fails the standard.
But there are also modifiers to this basic calculation:
If the marking is mixed capitals and smalls, then you would need to make sure that the smalls are 1.5mm (making the capitals around 2.0mm).
If the font is heavy bold or another font that makes the gaps between appear smaller (e.g. an E with the legs not evenly spaced) the size should be increase to make sure gaps are at least 0.3mm.
Finally, the marking should have good contrast and good quality printing (sharp edges), otherwise again the size should be increased to compensate. For example, if the printing resolution is limited to 0.05mm, then you would need to increase the size to around 2.0mm to make sure the gaps are 0.3mm.
All in all, it seems to me that:
<1.5mm is clear fail
1.5 - 2.5 is subjective and should be checked
>2.5mm is a clear pass
The standard mentions a range of 100-1500 lx. Obviously, the low end seems to be the worst case. The higher value of 1500 lx seems to be more important for indicator lights, as per IEC 60601-1-8 (alarm indicators).