Save the 900$ and wear out the tiagra, you could replace it twice for that amount of money or upgrade to 105 if you want the extra gear that would bring.

If you want to get someone to measure you, there's always the competitive cyclist fit calculator that can give you some numbers to use as a starting point for fit. For a random comparison I'm 5'10" but ride a 54cm roubaix since I've got short legs for someone my size.