I have not solved this myself so I can only speculate. I suspect that the input fish are limited to single digit ranges but that your own size can grow as large as the food eaten allows. Perhaps there is a pattern with regards to how much you eat and how big you get? Maybe we should consider ourselves a special fish that can gorow past the limits of all other fish? I realize that I am answering with more questions to a question, sorry about that.

This fork is unneeded. There was more recent code that I should have used as a starting point. Forking an old version to fix a warning is not very useful. I Don't know how to delete this thing.

I removed the string copying. It creates a warning. It should be fine to pass a string directly, given that strings are immutable in C#.

I completed this Kata just now. I noticed that the test code made a warning appear. The warning goes like this:
tests/Fixture.cs(22,20): warning CS0618: 'string.Copy(string)' is obsolete: 'This API should not be used to create mutable strings. See https://go.microsoft.com/fwlink/?linkid=2084035 for alternatives.'
Given that C# strings are immutable, the offending line is unneded. I decided to make a fork with that line removed.

Then I ran into an issue: the Assert.AreEqual, that the test code uses, was not working anymore. So, I altered the test code to use Assert.That instead.

The motivation for all this was to remove the warning that the test code generates.

It was my first time seeing [,] in C#. Before I have only seen [][] for multidimensional arrys. Thanks for the lesson.

Translated from C#. I'm new to F# so careful review recommended.

F# description tells to return Nothing if conversion is not possible. For F# it should read None instead of Nothing.