modify a .stl file

Dear all,I am quite new in the 3D design and I would like from the .stl attachment to:
- have a 4 mm lumen to allow air flow from this "Y" shape.
- have the proper size set:
> overall size of the object: 58 mm
> Size of the common arm: 30 mm
> Size of each branch: 28 mm
>lumen diameter: 4 mm
>outer diameter: 6 mm.
If someone could help me in the design that would be really niceAn other option would be to get this coded through openscad ?
Thanks a lot for your help here :-)
Best,
Arik