A Mullen mouth loose ring bit is a great choice for a gentle, simple bit which won't interfere with the horse's tongue and will offer a sure response. Rubber bits are extra beneficial for sensitive horses because the mouthpiece is more flexible, and the rubber increases chewing and salivation. The fixed mouthpiece ensures pressure on the tongue doesn't change significantly.
Ring size in size 95, 105, 115 = 45mm. Ring size in 125, 135 = 55mm.
Rinse with water and wipe clean after each use.