Hi,
DirectoryInfo.GetFiles() will return you FileInfo[] which is list of all files in given folder path. To get length (size) of file you have to use FileInfo.Length property which returns you length in bytes.
Here in your case you have to divide file length/1024 to get size in Kb.
Try with below code -
DirectoryInfo dir = new DirectoryInfo("folder path here");
var files = dir.GetFiles().Where(m => (m.Length / 1024) > 200);
Now,
files will hold list of files from your given folder having size greater than 200Kb.
Hope this helps.