Count words that are longer than a set Length
Signature:

public static int LongerThanStingCompair(string s, int n)


Compair is misspelled, as is Sting - they should be Compare and String, respectively. Although, a better name might be CountLinesLongerThan.

s and n aren't great names for parameters.

On to your code:

// splits based on nextLine break
var sCount = s.Split('\n');


The comment is unnecessary and slightly wrong. The code splits on all new line characters.

// Cast to list
sCount.ToList();


This comment is also incorrect - that doesn't simply cast it creates a new list from an array. It's unnecessary because you only need to enumerate the lines - you don't need the functionality of the list.

Here's a Linq based alternate for you:

public static int CountLinesLongerThan(string source, int characterLimit)
{
    if (source == null) 
    {
        throw new ArgumentNullException("source");
    }
    return source.Split('\n')
        .Count(line =&gt; line.Length &gt; characterLimit);
}