New Fitting - start the ductfittingadd command, double click on the fitting picture on the property palette. Choose your fitting and modify the sizes using the dialog.
Existing Fitting - select the fitting and double click on the fitting picture on the property palette and modify the sizes using the newly opened dialog.
If your having issues with adding fittings then i would suggest reading the help file on drawing ductwork. You can also download the tutorial from autodesk. It is for the 2010 version of mep and it is in metric but it should help you. The program has changed some since them but it is still worthwhile to go through it.
AutoCAD MEP Tutorial Download Link
![Autodesk Design & Make Report](https://forums.autodesk.com/t5/image/serverpage/image-id/1350519iCB55845EBFC43016/image-size/large?v=v2&px=999" "Autodesk Design & Make Report")
