* Length: A long, rambling speech is certainly larger than a short, concise one.
* Impact: A speech that deeply affects a large audience could be considered bigger than one that is ignored.
* Scope: A speech covering a wide range of topics might be considered bigger than one focused on a single issue.
* Audience: A speech delivered to a vast, international audience is larger than one spoken to a small group.
To help me answer your question, please clarify what you mean by "biggest".
For example, you could ask:
* "What type of speech is the longest?"
* "What type of speech has the most impact?"
* "What type of speech covers the most topics?"
* "What type of speech has the largest audience?"