Whenever you create a project in SketchList 3D, you're asked to specify the overall dimensions of the project and whether these dimensions are imperial or metric. If you choose imperial, then you should enter all your dimensions in inches; if you choose metric, then you should use centimeters throughout the project. You can select different units for different projects.
When you enter the dimensions for a project, assembly, or component, you only have to enter the numbers; there is no need to enter the units. SketchList 3D remembers the units you chose at the start of the project and applies them throughout the project.