How can I start the Windows Terminal maximized by default?

In addition to what @Petar said, since preview v.1.1.1671.0, --fullscreen and --maximized are two optional switches for the wt execution alias if you're automating something and trying to run the terminal maximized only sometimes.

So, these two ways of launching the terminal from the command line are now an option:

wt --maximized

wt --fullscreen

If you mean the new Windows terminal (preview), that is possible with adding:

"launchMode": "maximized"

to the "globals" section

Source: Windows Terminal Preview 1910 Release