Cut off the side branches and W&T at the height you want. Diameter looks good and it should grow well for you. Diameter doesn’t have to match perfectly although it can if you have wood the same size. Just ensure perfect alignment on one side and you will be golden.
Size difference can be really bad and it will be fine 
Wrap thinly and completely to seal up.
https://growingfruit.org/t/grafting-isnt-always-easy-to-big-for-whip-and-tongue-to-small-for-cleft-grafting-not-a-candidate-for-rind/45410/14?u=snowflake