Euglena are unicellular protists that have animal and plant-like characteristics. I share this quick video in my online bio labs (not personal content) and you can clearly see the red "eye-spot" that responds to light. It is in no way a "true eye", but one of the most primitive examples of a distinct light sensing organelle that influences locomotion.

https://youtu.be/rFCBrpgkmsQ

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An eye is a light sensitive structure that generally requires a brain to process the signals. This is because the eye is generally used as a way of finding food or evading predators.

A plant doesn’t move so it can’t evade predators and only needs to know which direction the sun is coming from so an eye would be an expensive organ to develop and own. However a plant can “see/sense” which way the sun is coming from and adjust itself in that direction or “curl up at night” so in a some form a plant must be able to “see light” even though it doesn’t have an organ we would recognise as an eye.

Poor little water bears without eyes. Who will give eyes to the Tardigrades? Oh wait. They DO have eyes? Whoa. I guess small eyes without lenses. Or eyespots at least. And not all species. They likely can only see in black and white and without lenses, it’s likely closer to the single-called organisms “sense of light”.

The smallest known insects are parasitic wasps in the families Mymaridae and Trichogrammatidae, both of which contain some species that are less than 0.2 mm long as adults. This is actually smaller than many single-celled organisms; here's a picture of one to scale with Parameceium and Amoeba (from Polilov 2012). Despite this, they still have fully functional eyes (or at least females do, males are often eyeless). Though their visual acuity can't be very good since they only have a few dozen ommatidia, each of which is something like 5000-6000 nm across, which is barely much bigger than the range of wavelengths of visible light. If you're willing to count simpler eyespots without any real image resolution capabilities there may be smaller contenders though, like tardigrades as previously mentioned, or maybe some flatworms.

As for your second question, there are many single-celled eukaryotes which can at least detect light in some way, though probably the most impressive eyes among these are found in certain dinoflagellates. Some species have structures called ocelloids, which are complex camera-type eyes composed of multiple organelles, including a cornea made of mitochondria and a retina formed from modified chloroplasts (Gavelis et al. 2015). Even some bacteria can be surprisingly good at detecting light though, with recent work showing that some cyanobacteria can effectively use their entire cell membrane as a spherical lens to track light sources (Schuergers et al. 2016).

All an eye is is a cluster of specialized, photosensitive cells. Many microorganisms (i.e. bacteria) who rely on light for photosynthesis are able to sense where light is and move towards it. In that way, some bacteria can be considered to have eye-like functions.

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