Self sealing bike tubes work great for tiny holes caused by small thorns or other sharps. I have been using them for years with my mountain bike. I change tubes about once per year and can see all the holes that have been filed and sealed.
I used to get a flat tire once per month but since I switched to tubes with sealant I can go 6 months without a flat.
Once thing to note is that they will not repair a major hole like if you hit a nail.
They cost less than $10 each and last me a year. They are available at retailers and online stores. I highly recommend you install them to spend less time fixing flats and more time riding.
There is no shortage of videos of mountain bike crashes from around the world, with many riders with appropriate safety equipment escaping serious injury, while others were not that fortunate. However, some of the most spectacular mountain bike crashes have been staged for television shows and movies, using stunt riders.
It depends on your height, leg length, and maker. While standing above the bike, you should have a few inches of clearance. If you are buying online, they will have detailed sizing charts. As an example, I am 5-10 and have a 17-inch frame. If in doubt get the next smaller size.