The max length is only 508 characters in Internet Explorer (all versions)

In Firefox, Opera, etc. you can have a 35,000+ character bookmarklet!

I know, cause I’ve written some 5,000+ character bookmarklets, and use them every day.

For the record, it is actually 512 characters in IE, but 4 characters are reserved for the prefix “url:” which is how Windows defines a shortcut.