Try something a little simpler:
string inputStr = "50x70CM";
string[] parts = inputStr.Split('X', 'x', 'C', 'c', 'I', 'i');
int width = 0;
int height = 0;
if (parts.Length >= 2)
{
int.TryParse(parts[0], out width);
int.TryParse(parts[1], out height);
}