How to calculate the maximum current rating of an ammeter manganin shunt?
Start with the maximum power dissipation, which is 5 W.

The first thing you need to decide is how hot you are willing to let the wire get, which you haven't told us.

Then you find the length of wire that can dissipate 5 W while staying within your maximum temperature rise spec.  This has nothing to do with the electrical properties of the wire.  It's mostly just a geometry issue.  You can probably look this up.

Once you have the minimum length of wire you need, find the diameter that gives you the desired 50 m&Omega; resistance over that length.  Round up to the nearest available diameter, and make the wire longer to compensate.