Git, and tools that use Git must support full-sized names with Unicode characters. Although you may encounter errors with some tools, there is no reason to compromise how you sign your work.
A good way to see what is generally acceptable is to look at an existing project. In Git itself, for example:
git log --pretty=format:%an | sort -u | less
In particular, a check for developers with a name longer than yours:
git log --pretty=format:%an | sort -u | egrep '.{25}'
shows seventeen longer names.
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