Thank you for your answer! At least it draws something.
Sadly it is always as big as the face you are creating it on, no matter how small you set the width and height.

RegisterCommand('decal', function (src, arg, rawInput)
local decalType = tonumber(arg[1])
if decalType == nil then decalType = 1020 end
local targetedCoord = GetEntityCoords(PlayerPedId())
testdecal = AddDecal(decalType,
targetedCoord.x, targetedCoord.y, targetedCoord.z-1.0, --position
0.0, 0.0, -0.301, --direction
0, 0, 0, --unknown
0.1, 0.1, --width height
1.0, 0.0, 0.0, 110.0, 150.0, false, false, false)
Citizen.Wait(1000)
print('Is decal alive', IsDecalAlive(testdecal))
Citizen.Wait(1000)
RemoveDecalsInRange(targetedCoord, 1.0)
Citizen.Wait(100)
print('Is decal alive', IsDecalAlive(testdecal))
end)