What gauge pick I should get for my acoustic guitar?

Try a few different gauges. Picks are cheap, so you can afford to buy a few different brands as well. I used to use a 0.73mm pick, but you need to find what works for you.

Although I would suggest something firm enough that produces a sound easily, but not too hard that you're always breaking strings.

A good question. I teach guitar and always start my students on a medium gauge pick. Heavy gauge picks tend to dig into the strings, they also have a 'hard' attack on the strings and extra light gauge are difficult to control. As you become more proficient you will have every gauge in your pick box. Picks do determine the 'attack' and feel in any given piece of music that you are playing. You will learn this a s you progress. keep on rockin'