How to compare Twitter bike size with other brands?

Different companies produce different bike geometries. It varies according to the model of the bike and its purpose.  The geometry of the bike would be expressed by different metric systems, e.g, the tire size of a mountain bike may be expressed by inches, like 29", but in millimeters in a road bike. Some companies describe their frames by letters, others by numbers, and those that use numbers may measure different parts of the frame. We strongly recommend looking at this website to compare bicycle geometries if you have a number in mind. https://geometrygeeks.bike/ 

Look at this example: 

As you can see, different companies have different geometries and use different measure patterns to describe the size of the bike.