Layout

Go to Preferences > Appearance and choose Single Panel (Gmail-style: the list fills the window and a conversation opens over it), Two Panel (list on the left, conversation on the right), or Two Panel Vertical (list on top, conversation below), then click Apply Layout. In the two-panel layouts you can drag the divider between the list and the conversation, and double-click a conversation to open it in its own window.

One line or three lines per message?

The message list picks its style from its width. Wider than about 540 pixels it shows one line per conversation with sender, subject, and date in columns; narrower than that it switches to a multi-line card with the subject and a preview snippet. To get the multi-line style, use Two Panel and drag the divider to make the list narrower; to get the columns, widen it. Themes can change the threshold.

Order of messages

Conversations are always listed newest-activity-first; sorting by sender or subject isn't available, so use search (from:someone) when you need to see one person's mail together. Inside a conversation, the oldest message is shown first by default. To read the newest reply at the top, go to Preferences > General > Reading and check Display conversations in descending chronological order.

Dates, names, and width

The same Reading section has Use 24-hour clock, Show first and last names of all recipients, Show full message headers by default, and Restrict width of messages to maximize readability, which stops long lines from stretching across a wide window.

Text size

The Scaling slider in Preferences > Appearance enlarges or shrinks the whole interface, including the message list and the text of the emails you read. It doesn't affect the messages you send.