Can't say about how to prevent them but the last one I had was about the size of an M&M. This stuff helped a lot https://www.amazon.com/dp/B00WOSEKP2
NAD, but I get these a lot too - they're just canker sores. Not serious other than being seriously annoying lolol
They can be triggered by stress, acidic foods, and a number of things (vitamin deficiency being among them).
I personally use this and this when I get them and they help a lot. Good luck!
I found it on Amazon Where are you located that it disappeared?