A video showing the use of an adjustable circular saw cut for installing a down-light: 

https://www.facebook.com/reel/829838691817543?fs=e&s=TIeQ9V&mibextid=rzpEBT

 

Source & Credit from above link:

Diameter of hole can be adjusted with this circular saw up to a certain length. But, good enough for most applications.

Related: Circular or Square Down-lights?

This resource is found in the above link

Which recessed ceiling lights should I install in my home? 

       OR  

Square lights or round ceiling lights?

Source & Credit:

 

1. Frankly, this has nothing to do with Feng Shui. Yes, based on Common sense.

1.1. There are some claims that square lights have higher luminous efficacy. However, there is no argument, here that both square and round lights are of a flat panel design, thus, they should be of equal luminous efficacy. 

2. Personally, I always prefer round ceiling lights. Why?

3. Majority of the light installers use a drill with a circular saw attached to it. And the diameter of the saw is just nice so that a circular/round ceiling light can easily be mounted once this hole has been drilled.

4. Contrast it with that for a square light.  You don't see a saw capable of sawing a square hole right?

4.1. To install a square light, a circular hole has to be drilled / sawed. Next, a workman (work-person) has to use a linear saw and start to prepare a squarish hole. And the quality of this square hole is at the mercy of this person. If this person is not skillful or had sawed a larger hole, this is what can happen as shown in the above photo. 

 

This is why I personally prefer this one:

That's Me!

What about you?

Seen a re-sizeable circular saw?