Controlling the width of the nivo slider using the class="class names" parameter to set the CSS class name(s) for the nivo slider.
Here we set the width to 50% using the class name “w50pc”.

Block editor alternative solution

It’s not necessary to do this using the block editor. The same result can be achieved using columns. Here each column is 50%. Don’t forget to remove the class=w50pc parameter from the shortcode.

See also

[div], [ediv]