Pump it up! Inflatable rubber plugs are great way to gently increase the size of what you can take.
These inflatable plugs by Mister B are of the best quality. The rubber is approximately 2 mm thick, they're glued together very carefully and have high quality metal valves.
 
.As for any insertable toy: use lubricant. Because these inflatables are made of natural latex: do not use oil based lubricants because these will deteriorate the rubber.
 
Size (length x diameter in mm):
M: 160 x 35 mm