You can find this stuff out by simply starting a blank .capx, adding something to it, and exporting, then check the file size. I was curious though, and did it for you.
- For example, is it better to use jpg where opacity isnt needed?
Can be, jpgs can be smaller than pngs if the conditions are right.
- Is it better to chop art up into small pieces, and use as few different sprites as possible?
Chopping a big picture into smaller pictures won't help the file size.
- How big impact does the number of events have on the total project size?
Not much. 10 events, with 50 conditions and 50 actions total, added 11k.
- How big impact does the different "objects" (like touch, keyboard) have on the total project size?
Again, not very much. Adding a unique text object to a project adds 8k to the exported project. A sprite, with a 1.5k image added 23k. The images and sounds generally take up far more space.
- Should i make all art assets 1:1 in size, to reduce file size? What impact could this have on portability/scaleability?
The ratio shouldn't matter for file size. Could matter for vram use.
- Does cloning objects/art assets duplicate the file, meaning double the size used? Or does it work the same way as copy/paste?
Cloning the 23k sprite added 2k - almost the entirety of which went to duplicating the image.