When I run a long command w just shows part of it by default.
How do I make it show the full command?
The command is designed to produce a columnar layout, and for that reason truncates all output that exceeds the character width of the current display. Otherwise, it would be wrapped and render the output less structured. In a larger terminal window, e.g. after you maximized the terminal, more output will be shown when running the command again.
A trick to see all output wrapped within a smaller window, but loosing some of the column formatting, would be to pipe the output to cat:
w | cat
tee or more would also generate similar results.