I am 5'9" and my wife is 5'5". If my wife sits behind me, it is comfortable for both short and long trip. If another person of my size sits behind me, it is comfortable definitely for short trip. But, it may not be comfortable enough for long trip (say more than 5 hour driving). X1 is definitely too small to seat four persons higher than 6'.
My friend has a last generation 3 series (2009). He especially compared my X1 with his 328i and said X1's rear seat is larger than that of his 328i.