Well, the human eye can see visible light, which is around 900 nanometres, so if I had to guess (although I’m not sure) you would need about 900 nanometres of atoms (in a blob), which is quite a few. Maybe like 10 million million? (your brain probably wouldn’t know what it was looking at)
The eye is remarkably sensitive. The minimum number of photons (particles of light) that the eye can detect in a short flash is around 100. This is equivalent to about a trillionth of a Watt! Compare this to a 40 Watt light bulb and you’ll agree it’s a fantastically small amount of light!
In terms of size, the smallest thing the eye can see is a few thousands of a millimeter across. Amazing!