URL最长是多少?

What is the maximum length of a URL?

RFC 2616 (Hypertext Transfer Protocol HTTP/1.1) section 3.2.1 says

The HTTP protocol does not place any a priori limit on the length of
a URI. Servers MUST be able to handle the URI of any resource they serve, and SHOULD be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server SHOULD return 414 (Request-URI Too Long) status if a URI is longer than the server can handle (see section 10.4.15).

Note: Servers ought to be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations might not properly support these lengths.

That's what the standards say. For the reality, see this research over at boutell.com to see what individual browser and server implementations will support. It's worth a read, but the executive summary is:

Extremely long URLs are usually a mistake. URLs over 2,000 characters will not work in the most popular web browser. Don't use them if you intend your site to work for the majority of Internet users.