I'm a new cyclist and I'm looking for a new bike primarily for recreation and also for fitness. I would like a mountain bike mainly because I like the style and the fat tires! However, I won't really be doing much if any trail riding. I did some research to try and determine the right size bike for me and based on my 5'9" height and my 29" inside leg I found that I needed an 18" or medium sized mountain bike. However, when I went to the store to check them out, it seemed like the 18" or medium bikes were quite small, and I felt like I wouldn't be comfortable on something like that.
So I was wondering if mountain Bikes, by design, are somewhat smaller than road or hybrid bikes due to the way they are ridden? If I'm not intending to do any actual mountain biking with it, would I be OK buying a larger size? I'm assuming that when riding trails etc. most riders are standing in the pedals rather than sitting on the saddle, but I'm guessing I'll spend more time in the saddle when I'm riding, which makes me feel like a larger bike would be best for me.
Just looking for some guidance here so I don't make a bad decision. Thanks