Ever since the first design: http://www.thingiverse.com/thing:590921 I wanted to design an as parametric as possible tricopter frame and accesories, and I finally did. Now anyone can introduce its own variables and generate a tricopter that fits ;D.
It doesn't matter what size of boom you want to use, neither the tail servo, just introduce your characteristics and you'll be fine.
1/ Open the .scad file you need.
2/ Put your own values. Remember to set the "printable" value to true.
3/ Uncomment (by removing the marked "") the piece you want to generate.
6/ Have fun ;).