The comment field in the mockups appears to be smaller until it's required to be larger, and this would appear to be done automatically. I think that since the message is limited to a fixed length of characters, the initial and only size of the textarea should accomodate that amount of text (on average, since typing 140 "W"s takes more space than 140 "i"s using a variable width font). Twitter initially collapses their textarea (presumably to conserve screen space) but enlarges it on focus. Identica present the user with a textbox that is initially large enough to fit an average 140 character message, and it does not resize at all. Initially presenting the user with a textbox that can handle 3 to 4 rows of text also sets the users's initial expectation of how long the message is allowed to be - encouraging them to be more verbose, which hopefully translates into more specific and descriptive.
This post was posted by Trevor Parscal (WMF), but signed as Trevor Parscal.