Smaller volume of water changes temperature faster than larger volumes.
In the summer it can overheat very easily.
Partial shade, such as the dappled light under a tree will help a lot. Early morning sun or very late in the afternoon is OK. If it still gets too hot you may have to add some shade cloth.
Digging it into the ground is very good insulation, and can help with cooling, especially if it is in an area where the soil is moist, like a flower garden.
The more water you can expose to the air for evaporation the cooler you can get it. Think about a small stream and waterfall.
In winter too small a container will freeze solid. No fish can handle that. As for heating it, there are a few things you can do, but essentially it comes down to spending money to heat it. Some ideas:
1) Insulate it by installing foam around it. This will help protect it from the cold earth, but you lose the summer cooling. Float a sheet of Styrofoam on the surface.
2) Build a mini-greenhouse over it. This can be as simple as some hoops stuck in the ground then drape some clear plastic over it. Depending on how much snow you get you decide how strong to make the hoops. Make sure the plastic sheeting is sealed at the ground so no air blows in. You could make an insulated 3-sided box, leaving the roof and south side to cover with glass or clear plastic.
3) If your area is mostly sunny in the winter (just cold) then look into solar water heating like they do for swimming pools or to pre-heat the water going into your water heater. You would need insulated pipes and a recirculating pump. Too many cloudy days will make this not work very well.
4) Stock tank de-icer will keep a small spot on the surface defrosted so the fish can get oxygen. Add a pump near the bottom of the pond that pumps the water upwards, and in a small container the stock tank water heater might be enough to keep the whole pond a bit warmer.
Drain the plumbing, bring the fish and plants into the house and overwinter them in an aquarium. In the spring, refill with warm enough water and test it to be sure it stays warm enough before moving the fish outside.
Note that some fish are not very pretty when viewed from the top. This is to hide them from predators. Koi and Golds have been bred to be viewed from the top. Platies are OK, you will still see some color. They will handle temperatures into the mid 60sF. Guppies are almost impossible to see from the top. As colorful as they are from the side, they are still grey on the top.