How do you know what size of bike you should buy?

A good place to start is using the sizing charts included for each bike on the info page. If you're in the middle of the recommended height range for a specific size then usually that's the correct one to go with. If you find yourself overlapping sizes at either the top or bottom of the height recommendation than sizing up or down could be a consideration.

For more relaxed fitting "city-style" bikes sizing is less crucial because often they will have adjustable stems and offer more overlap in sizing. Smaller bikes will be slightly easier to manage when starting and stopping while larger bikes will feel more stable when riding. Most often we recommend sizing down in this category.

Bikes that have a more aggressive riding position i.e. road, gravel and mountain bikes sizing is more important. In these categories we usually look closely at 3 numbers, reach, stack, and top-tube length.