Azure DevOps set MultiLine Row parameter

Is it possible to set the multiLine row parameter in the task.json file for a custom task?

It's possible. Use .net core task have a textbox Path to projects:

Corresponding html see here:

Which is default rows of MultiLine type.

So I compared the source of Use .net core task and PS task here and found:

       {
            "name": "projects",
            "type": "multiLine",
            "label": "Path to project(s)",
            "defaultValue": "",
            "visibleRule": "command = build || command = restore || command = run || command = test || command = custom || publishWebProjects = false",
            "required": false,
            "helpMarkDown": "The path to the csproj file(s) to use. You can use wildcards (e.g. **/*.csproj for all .csproj files in all subfolders)."
        }

and this:

        {
            "name": "script",
            "type": "multiLine",
            "label": "Script",
            "visibleRule": "targetType = inline",
            "required": true,
            "defaultValue": "# Write your PowerShell commands here.\n\nWrite-Host \"Hello World\"\n",
            "properties": {
                "resizable": "true",
                "rows": "10",
                "maxLength": "20000"
            },
            "helpMarkDown": ""
        }

The first script comes from dotnet task and second one comes from PS task. They both use MultiLine type.

According to the difference between these two scripts I think you can get what you want by setting the rows element in properties element. Something like this:

    "properties": {
        ...,
        "rows": "xxx",
        ...
    }

Hope it helps and if I misunderstand anything, feel free to correct me :)