Find the longest item in array of strings in MongoDB

A have a collection Products in MongoDB:

{
  "name": "My Product 01",
  "description": "This is an excelent product", 
  "tags": [
    "AA",
    "BBBB",
    "C",
    "DDDDDDDDDDD"
  ]  
}

How can I find the single longest occurrence if an item in the whole collection? (if I had only this object, the longest occurrence would be "DDDDDDDDDDD" with 11 characters).

$unwind to tags level. Use $strLenCP to compute the length. Use $setWindowFields to $rank the length. $match rank: 1 to select the one(s) with longest length.

db.Products.aggregate([
  {
    "$unwind": "$tags"
  },
  {
    "$set": {
      "length": {
        "$strLenCP": "$tags"
      }
    }
  },
  {
    "$setWindowFields": {
      "partitionBy": null,
      "sortBy": {
        "length": -1
      },
      "output": {
        "rank": {
          $rank: {}
        }
      }
    }
  },
  {
    "$match": {
      rank: 1
    }
  }
])

Mongo Playground

db.Products.aggregate([
  // Unwind the tags array
  {
    $unwind: "$tags"
  },
  // Project each tag with its length
  {
    $project: {
      tag: "$tags",
      length: {
        $strLenCP: "$tags"
      }
    }
  },
  // Sort by length in descending order
  {
    $sort: {
      length: -1
    }
  },
  // Limit to the longest tag
  {
    $limit: 1
  },
  // Project only the tag field
  {
    $project: {
      _id: 0,
      tag: 1
    }
  }
])

generates:

[
  {
    "tag": "DDDDDDDDDDD"
  }
]