Currently the "EXTRNLNKS" and "TRACKBACKLINKS" tables use a VARCHAR 255 data type for URL and and LOWERURL columns.
It is requested that Confluence would support longer URLs in these tables and any others that store URLs.
This can break imports if they contain URLs greater than 255 characters.
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).
That RFC has been obsoleted by RFC7230 which is a refresh of the HTTP/1.1 specification. It contains similar language, but also goes on to suggest this:
Various ad hoc limitations on request-line length are found in practice. It is RECOMMENDED that all HTTP senders and recipients support, at a minimum, request-line lengths of 8000 octets.
The sitemaps protocol, which allows a site to inform search engines about available pages, has a limit of 2048 characters in a URL.
There's also some research from 2010 into the maximum URL length that search engines will crawl and index. They found the limit was 2047 chars, which appears allied to the sitemap protocol spec. However, they also found the Google SERP tool wouldn't cope with URLs longer than 1855 chars.
Below are character limits for various browsers:
Practically speaking, supporting the storage of URLs with length < 2048 characters should be sufficient (VARCHAR 2048).
Knowledgebase article tracking the associated import error: Space or Site imports fail when URLs in import file are longer than 255 characters