Continuing this series thanks to a prod from user KS on the Field Limits doc:
Does URL field type have a character limit?
Indeed it does: 255 characters. And — unlike with Integer fields — this restriction is in sync with the SFDC field type of the same name, which also is limited to 255.
This wouldn't be a problem, save for the fact that Marketo can't truncate longer URLs (including pathnames and query strings) to a guaranteed working URL.
See, an implicitly-fully-qualified domain name like www.example.com or pages.en.example.co.uk— not including any leading or trailing URL components like https:// or the /path/name?query=string—must indeed be under 255 characters. (In fact it must be under 253.)
So if you're only storing pre-parsed domain names, there's no problem with the max field length.
But if your source could be a full Referrer URL, or a long URL from your product catalog, it's certainly possible to go over 255 characters.