Improve new window & letterboxing dimensions

Meta issue for our various letterboxing issues we should fix in 13.0 stable:

• Change the default start window size from 1000x1000 mullvad-browser#175 (moved) (duplicate of the below ticket)
• Increase max width of new windows #33282 (closed)
• Re-evaluate letterboxing dimension choices #30556 (closed)

Let's plan on doing this work in base-browser so tor-browser can get these improvements also.