To be honest, for an one-off machine, plans are probably overkill and expensive. A couple of light beams/sensors to set min/max length acceptance criteria and the digital scale both as inputs to a programmable logic controller. The output mechanisms would be two pneumatic pushers to either slide the round into the good or the rejected bin. It is a semi-automatic system as it requires a user to insert each individual round into the testing station unless you wanted to try and design a controlled-feed hopper that could reliably place the round correctly on the pan (much more involved). Also, you would have to establish separately whether each round was rejected for weight or length issues if important.
The only mechanical questions with the system as described that I see are:
1) whether the deflection variation of the digital scale pan between min and max weight would affect significantly the apparent length of the round to the light beams mounted independently of the scale.
2) if the light beams are mechanically linked to the scale pan to better control the length, whether the additional weight on the pan would affect the scale's accuracy
Both of these are pretty straight-forward to a sharp engineering undergraduate student, and the electrical side is pretty straightforward as well. If you decide that you want plans or wiring diagrams without high cost, you might be able to put up fliers at the engineering department in a local university to provide a side project for some student. Of course, they could probably build it as well. Otherwise, prototyping and design companies exist. I can recommend a few good ones out here in the West or one in Wisconsin if you wish. They are reasonably-priced compared to their competition; however, the cost will be significantly higher than the student route.