It is best to lay the model flat vs to print it standing in most cases
Overhangs can be problematic in models. Think about how the model extrudes layer by layer. If something it really far from it's center of mass, the print might need supports to actually print. See the Cura manual for more info <https://ultimaker.com/en/resources/20406-installation-cura-2-1>
The print must fit in a 20x20x20cm cube. It is best to lay the model flat vs to print it standing in most cases. Overhangs can be problematic in models. Think about how the model extrudes layer by layer. If something it really far from it's center of mass, the print might need supports to actually print. See the Cura manual for more info <https://ultimaker.com/en/resources/20406-installation-cura-2-1>