The box you type your post into, and also the same box you type your message into for the Private Message system has been expanded vertically.

For posts, you can expand it further, but cannot shrink it past the default height.

For private messages, you cannot expand or shrink it at all. This may have an effect on the "display" of the prior messages in the "conversation". If this is negatively impactful, please voice your displeasure

The change was made because w/o the ability to expand it in the PM system, it was very hard to type longer messages especially those which used bullet lists.