Heating a room with an open staircase

I am trying to size a stove for a room with an open staircase. The room measures 6m x 4.6m x 2.4m. It has 3 external walls. External walls are made up of 100mm concrete block - 100 platinum bead full fill insulation - 100mm concrete block. All walls are plastered with scratch & skim. All windows/patio doors are taped in. Windows are fitted with trickle vents. There are no vents in the walls. Staircase rises to a wrap around landing.

Wall 1: Patio door 2.4m x 1.8m u-value 1.4 West facing
Wall 2: 2 x casement window 0.8m x 1.3m u-value 1.4 South Facing
Wall 3 (internal): Staircase rising to 1st floor & "standard" doorway to next room
Wall 4: Solid

Any calculators/calculations I can find are for a "standard" room (ie with no staircase in the room).

Can anyone assist?

You should calculate to take into account losing heat to the upstairs landing, thus sizing for the room and the landing.

You must install vents in the room for the stove unless it is a room sealed stove/balanced flue stove. The vents should be sized in accordance with the kw output of the stove and in accordance with M.I.'s. Inadequate ventilation can lead to incomplete combustion and carbon monoxide poisoning.

have you carried out an air-tightness test?