Copied from http://stackoverflow.com/questions/24792974/laravel-4-migrations-schema-index-length

..create these indexes manually using DB::statement(); in your migration.

In your up() do something like this

DB::statement('CREATE INDEX description_idx ON Customers (description(100));');

And then in your down() you can simply

Schema::table('Customers', function($table) {
    $table->dropIndex('description_idx');
});

You can use DB::raw() as well

public function up()
{
    Schema::table('Customers', function($table) {
        $table->index([DB::raw('description(100)')]);
    });
}
public function down()
{
    Schema::table('Customers', function($table) {
        $table->dropIndex([DB::raw('description(100)')]);
    });
}