AFAIK anything up to 2000 characters ought to be supported.
Reproduction:
var s = "http://www."
+ string.Join("", Enumerable.Repeat("a", 249))
+ ".com";
new Uri(s, UriKind.Absolute);
One letter shorter one is being parsed with no issue:
var s = "http://www."
+ string.Join("", Enumerable.Repeat("a", 248))
+ ".com";
The Uri class code was originated in 2003 and is carried over to .net standard / core. Does MS still claim 255 characters for path / URI is enough for everybody's needs?