I believe Feynman addressed a similar point in one of his autobiographies. A magician could get a person to lead them to a specfic place just by holding their hand lightly and jiggling slightly. When they moved in the wrong direction the person would resist slightly more, so the correct position could be determined. This is effectively the ideomotor effect moving an entire person, so a claim that it can't move large things is silly.
For a ouija a small ideomotor effect in one person can cause the other people to react to it and move in that direction, conciously or unconciously. The ideomotor effect does not have to move the whole weight of three other peope's arms, it just causes them to move themselves.