Variable Size of Rectangle Dependant Height

Hi,

I have a tree like structure that is made with a combination of lines.

My first question is: how can I vary the width (x size) length of a rectangle that is location at each end point of a line? With the width decreasing the further it is away from the floor plane. (further up the z axis).

Next is a problem I have with joining the curves that have been produced. Not shown in the definition uploaded, but if I use 'join curves', it doesn't give me a result of one line, is this possible?

Thanks for the help.

Nate