What term refers to something that is a statement of opinion and is generally straightforward and short?

A statement of opinion which is generally straightforward and short is called an assertion.

When you assert something, you are expressing how strongly you believe it to be true. Since it is only an opinion, it does not mean that what you are saying is actually correct, but simply that you trust it to be. That is why it is often straightforward and short: you do not feel the need to justify yourself, and you want your statement to be impactful.

In a speech or a dialogue, it can be a rhetorical device aimed at appearing confident or unwavering in the eyes of your listener (s).

An example would be: "The work will be completed by Wednesday."