Is there a cognitive biasing effect that makes people believe that long statements / expressions / articles / books are more truthful than short ones (assuming that they are equal semantically)?
Message length is a peripheral cue in the elaboration likelihood model. This means that a message's length affects the likelihood that its recipient will be persuaded when the recipient is not scrutinizing the message's content attentively. When a message is evaluated through peripheral attention instead of central focus, simple heuristics that are easily fooled determine the extent of persuasion that results.
Petty and Cacioppo (1986, part 1) cite their earlier study (Petty & Cacioppo, 1984) as demonstrating that more arguments in favor of a message increases attitude positivity toward the message when the audience is not personally invested in understanding it even if the arguments are weak. This doesn't work when the audience is highly involved – then a greater number of weak arguments only makes attitudes worse.
Petty and Cacioppo (1986, part 2) also cite Wood, Kallgren, and Preisler's (1985) study, which persuaded its relatively inattentive participants more strongly with long messages than short messages. Again, this effect did not vary with message quality, but disappeared among participants who paid closer attention.
@Eoin recommended Heit and Rotello (2012), who replicated the results of Wood and colleagues. In fact, they even found the effect of message length among participants who were told not to judge arguments by length. This also occurred when listening to verbal arguments. Good stuff...
It seems like a casual version of the illusion of validity, but the illusion of validity is a more general bias that additional data generates additional validity. It's often used more in a lab setting then a debate setting, where an additional experiment may be included to lend support to a hypothesis, but the experimental outcome isn't actually surprising given the results of other experiments.