Provided that the radiator you opt for delivers an adequate heat output to warm the space, you can choose whether to have a horizontal or vertical radiator in your chosen room.
Typically, vertical radiators will prove an excellent option for compact rooms where space is at a premium, considering horizontal radiators usually take up a lot more wall space.
Essentially though, the choice between vertical and horizontal radiators will be based on what fits your personal preferences, and looks and fits best within your specific room.
Be sure to use our BTU Calculator to guarantee that any radiator you select, vertical or horizontal, offers a strong enough heat output to warm your room to a comfortable temperature.