Flexible Content works in much the same way as a repeater field, where you can check to see if there’s anything in it, and then reference all of the sub_fields within (note: sub_field).
In your case, once you’ve performed your initial check for existence, grab the select sub_field and check if there should be one or two columns:
<?php if(get_sub_field('columns_required') == "one_column") : ?>
<div class="column wide">
<?php the_sub_field('column_content'); ?>
</div>
<?php elseif( get_sub_field('columns_required') == "two_columns") : ?>
<div class="column half_size">
<?php the_sub_field('left_column'); ?>
</div>
<div class="column half_size">
<?php the_sub_field('right_column'); ?>
</div>
<?php endif; ?>
Hope that helps
