So I just put this little spike together:

```
public string MergeParameters()
{
    var parameters = new List<string>();

    if (!String.IsNullOrWhiteSpace(In))
    {
        parameters.Add(String.Format("in:{0}", In));
    }

    if (!String.IsNullOrWhiteSpace(Size))
    {
        parameters.Add(String.Format("size:{0}", Size));
    }

    return String.Join("+", parameters);
}
```

You still have the whole "is it empty?" check but I think it reads a bit better overall...