Is there a way to only specify only a minimum length? Perhaps another
attribute i can use?
Using the standard data annotation, No. You must specify the MaximumLength. Only the other parameters are optional.
In such a case, I’d recommend something like this:
[StringLength(int.MaxValue, MinimumLength = 7)]
You can also use a Regex (regular expression) attribute like this one:
[RegularExpression(@"^(?:.*[a-z]){7,}$", ErrorMessage = "String length must be greater than or equal 7 characters.")]
More on this here: Password Strength Validation with Regular Expressions