What size bikes have you been riding?

There are really 2 parts to a bikes size. The seat tube length, and top tube length. Too short of a seat tube and you will have to jack up your seat so much and that will raise the saddle to handle bar drop/reach. Too long and you won't be able to straddle the bike. Too long of a top tube and you will be stretched out too much to reach the bars. Too short and you will be too compressed.

Make sure you know how to set a proper saddle height (heel on the peddle, and when in line with the seat tube your leg should but pretty much fully extended). Then go by the amount of seat post showing, and should be close to the size of the head tube (non sloping top tube, if sloping use your imagination). Then you are good to go and can play with stem size/angle and handlebars to fine tune. Doesn't have to be exact, but glancing at it could give you a decent idea if you need to go a size bigger or smaller on the frame. At least for your 1st bike.

Bike sizing is different for everyone. Some people like the saddle a bit higher, some a bit lower. Some like a bigger frame, and some like a smaller frame. And some people have owned a bike for 2 years and don't even know what size it is. Ignore those peoples suggestions like the plague.