https://howtobeaconartist.tumblr.com/ Alll sorts of resources here.
I live in the US and I personally see 11x17, 8x11 (rarely), and postcard 4x6/5x7 prints for posters and things. You want AT LEAST 300dpi and +.25 on each side of whatever your target paper is. There is nothing wrong with doing an 11.5x17.5 image and resizing to just a postcard print; it'll still be very clear. You want that wiggle room for cropping/alternates, and also overprinting.
Everything else is "find out your target size" and if it's small, do 600dpi (it's way better to scale down than scale up) with a little bit of crop/wiggle room. Charms, buttons, and other small printables will almost always have a template file if you're getting them printed wherever. You can also find templates for basically any printable size image, really, with safe zones / crop marks that will help with the above cropping, too.