When it comes to mountain bike sizing, each brand will have its own size chart that you can consult, and usually, brands come in standard sizes: small, medium and large.
Sizing is often correlated with height, but if you think that you may be between sizes, it is usually best to go for the smaller size. It is easier to make adjustments in other areas that will help compensate instead of having a large frame that can’t be controlled. There are a few things that you can test when it comes to determining if a bike is the right fit for you. Measuring the Effective top tube is a great way to show how long a bike will feel when you are sat on it. You should measure the horizontal distance from the seat tube to the head tube.
Measuring the reach is also a good indicator of size for an MTB. To do this, you should measure the distance from the centre of the bottom bracket to the centre of the head tube using a horizontal line. This will help you measure the length of your bike and will give you an indication of what it is like to ride when out of the saddle.