I just got to the kanji for "sayeth," which is the same shape (and also serves as the basis for) the primitive for "tongue wagging in the mouth." Heisig says that the distinguishing factor is their size...is there that much difference? Which one is bigger than the other?

[Katsuo]

Comparing "day" 日 and "sayeth" 曰:

In most fonts "sayeth" is drawn than wider than "day" and often less tall as well. Also the centre horizontal stroke stops just short of the right vertical. (But sometimes "day" is written that way too.)

Context will usually confirm which it is.

Like Katsuo said (Katsuoが曰く), it depends on font. Sayeth often feels bigger, but is mainly wider. The middle stroke is also generally not connected, making it pretty easy to distinguish.

Don't worry about it, 曰 is used very seldomly. I've only ever seen it in the example above, used on slashdot.jp when quoting someone.