This all depends on the scale of the dolls house in question, generally these are given in ratios. The most common scale is 1:12 which equates to 1" represents 12" of 1 foot a female doll would be around 5 1/2 inches talls, doors around 6" and ceiling height probably around 8".
Another common sizes is the "barbie" scale which is 1:6 where 1" equates to 6" this makes the Dolls house quite large and is more suitable for younger children.
Other scales exsist such as 1:24 (generally known as half scale), 1:44 and 1:148 these smaller scales are prefered by older and adult dolls house collectors due to there minature size.
Hopefully this will give you some idea of the scale a painting should be for a dolls house.