Some pipe fittings dimensions are everytime equal but some not. For example tee(pipe fitting) family in the picture all dimensions may not be equal you can change different dimension for only third connection(inegal tee). But some tee’s dimensions are equal everytime(equal tee). Manufactures are made in this way.
When we connect fittings to pipe and change fittings nominal diameter parameter revit get me an error and can’t resize fittings. (This is only inegal tee fittings, equal tee nominal parameter has work). if i can change diameter parameter from modify tab i think it will be work. I am sizing pipes automacially according to flow but i can’t do it for only inegal tee fittings. I hope you understand me 
