12mm will be completely fine. Also if the wall plug is bent/warped after your first few attempts you might have to get some new ones. Can be a real pain to get deformed plugs into correct holes. Head in to the big green shed and get something like these. You can probably find some cheaper ones, that was just the first one that showed up on Google. Not bad to keep some of these around if you have a brick house.

https://www.bunnings.com.au/ramset-12-x-60mm-anchor-ramplug-15-pack_p2260095

Also make sure you're using masonry bits, and if your masonry bits are cheap/dull the hole you're drilling will likely be smaller than it should be. So either get a new sharp set, or you can just kind of rotate the drill bit around in the hole while moving in and out to hollow it out the extra mm.

Edit: also if you've got a cheap hammer drill and masonry bits. My advice is to start with a small hole 4-5mm and then use step up to the bigger bits. It will be much easier on the drill, bits and your arm.