Magento 2 Add "VARCHAR" type field into custom table

If you are using:

->addColumn(
    'your_column',
    \Magento\Framework\DB\Ddl\Table::TYPE_TEXT,
    55,
    ['nullable' => false, 'default' => '0'],
    'Demo'
)

It will return a column it your dabase a column: your_column, type varchar(55),NOT NULL, Default 0

If your column has more 255 characters, you need using:

  ->addColumn(
    'your_column',
    \Magento\Framework\DB\Ddl\Table::TYPE_TEXT,
    '2M',
    ['nullable' => false, 'default' => '0'],
    'Demo'
)

Return a column: name your_column , type mediumtext,NOT NULL, Default 0

there is No keyword called "VARCHAR" if you want to create a field with varchar you need use "TEXT" with length if length is not passed then it will consider as text filed

$columns = [
                 'custom_sku' => [
                  'type' => Table::TYPE_TEXT,
                  'nullable' => false,
                  'LENGTH' =>255,
                  'comment' => 'custom sku',
               ],
      ];

or

->addColumn(
        'summary',
        Table::TYPE_TEXT,
        255,
        ['nullable' => false, 'default' => '', 'LENGTH' =>255],
        'Summary'
    )

if it work accept the answer so it will use full for others