Last night we went on a discussion about resolution, what is dpi, lpi , spi, ppi. There were many things i got forgotten in terms and concept, but you can read more about it here. I recommend you to save this document on your computer as a reminder.
I use the 1.5x rule, so my work its usually at 300 ppi. Pay attention to photoshop and any software is it wrong to call it dpi, because its not dots per inch, its actually ppi, pixels per inch, i was surprised that ive never took attention to this always thought it was dpi, or dpis like i call them.
I like to work in A4 and A3 size. probably from 3000 to 6000 pixels wide or sometimes even more like 6500 by 300 ppi, that its wider than a A3 size paper.
Also while changing the ppi, the pixels wont change until you change the option to inches or other measure units like cm, i recommend you to not do that, what ever you do dont change the pixels of what you are already started, so starting a picture 1.5x bigger in a 300 ppi its a good way to start.
Higher canvas needs less ppi, because you need to see if from far view, i used to work on billboards, so you will be surprised how much pixelated the images were, we printed images of 900kb with many feet long and still look nice from far view.
Its like when you buy a bigger 80 lcd screen, you think you will see it bigger but its not, you have to move farther from the screen, which its good for the eyes, keeping things at distance its less eye strain and with more comfort, thats why you cant put a 80 inches tv screen on a real tiny room.
Try watching a movie in a movie theater 5 steps away and you will notice how bad the quality is, and how much trouble for the eyes to get the the main image.
Changing the ppi of the image wont affect the pixels size, but will affect the printing size, in photoshop do a right click with the magnifier and click on printing size. It will change, but the 100%of the image size will remain the same, unless you changed the inches of the canvas size, which i dont recommend changing it.
Corel painter doesnt have the viewing printing option.
Dont break the head with ipp, all the time the companies you work on wil give you the resolution they need for their printers.
If you want a poster, you dont need to have a bigger file, if you started at a 1.5x or a 2x size, you just need to change the ppi to match the printing specifications you actually need.
Hope this helps you clearing up your ideas.