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PHP, 195 bytes

Added the > 2 character restriction:

<?$a=$argv[1];$r=substr($a,-2,1)=="\n"?strrev($a):$a;foreach(explode("\n",$r)as$l){$s=strlen($l);
$x[$s]=++$i;$m=$i==$s?T:M;}$z=count($x);echo$i*$z>2?$z==1&&key($x)==$i?S:($z==1 && $i>2?R:$m):M?>

The upside down triangle adds an expensive 56 bytes to this!

Outputs are S,R,T,M

Saved a few bytes thanks to Dom Hastings. Wasted a couple to make it shapely ;)

Try it online!