Anyone know what size Cox Medallion this muffler is for?

Yours most welcome, my caliber is just hanging there if snugged it should read the same.
Well you just never know when you might pick up a .15, it might be worth hanging on to and source the .15.
I appreciate the offer but Im good to go on my end.

A .15 size engine would be more to run fuel wise the upswing is there are many park flying slow flying ARF RC planes still on the market that could be powered by the .15 making for a larger stable plane to enjoy without the cost of using a even larger fuel thirst engine. I hope you mend well with the hip surgery with a good regiment of food, vitamins and meds should speed things along as best as possible.

Also other options besides the Cars would be an Air Boat on wet grass might be some fun. A fishing rod and some water will allow you to throttle about and should it run out of fuel before you dock the boat you could cast over the hull and bring it back to shore. TD .09 size and .15 size have enough power a reliability throttle wise to insure success on your outings. Other option would be a hover craft splitting the blast for the skirt fill and some for the rudders for steering and forward motion. One other option is good ol round the pole flying with servo on the throttle and elevator perhaps even retracts, flaps and lights.