Actually, it is not as hard as it seems. I found out how to rescale particles after some trial & error and after I read Tacoman's guide about changing colors of particle systems (you'll find a link later). Now, let's get to it already, shall we?

First things first, you'll have to add a particle system to you project (obviously).

Now right click on it in the Animation set editor and choose "Instance particle system".

Then right click on the particle system again and hover to the "Edit particle system definition".
Certain particles are actually not "solid", they are made from parts, just like your scenebuild is made from props for example, so there you will see a list of these parts. You can change any of them separately. Just click on the desired one.

A new window should pop up called "Particle system editor". In the bottom-left corner you should find a list of options named "System properties". Click on "Radius Random".
To the right you should see a bunch of attributes and values. Scroll the list on your right to the bottom. Find attributes called "Radius min" and "Radius max". Play around with them (mostly with the second) to resize your particle.

When you're done, click "Accept".

As you probably noticed, your particle acts... weird. Only part of it seem to be resized. Well, do you remember that list of parts that pops up when you hover over "Edit particle system definition?"Yeah, that one. Editing values for each part will allow you to fine-tune the look of your particle system.

Well, that is basically it. This guide is not perfect, obviously, and I myself discovered this scaling thing literally today. I cannot really say how exactly do those values work, but at least I and you now know how to change them. If any of you have something to add to this guide, then leave a comment! Your help will be appreciated. Anyways, thanks for reading, I hope it helped.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=655184159 - Tacoman's "Customizing particle colors" guide

One another thing to mention: it may be worth to play around with "Radius scale - Radius_end_scale / Radius_start_scale". I don't know how exactly they work, but experimenting with these values may help you to tweak the particle system even further and possibly make it look better. Give it a go.