5'6"1/2 is plenty readable to me. That's 1689mm. Both are 4 digits long. I don't see a clear advantage of using mm.
There are advantages if you are ever doing any calculations on that number, say divide it by three for example.
Also, since you were fine in rounding the distance to half an inch (~12.5 mm), why would you not round the metric number to whole centimeters (10 mm)? That would make it a nice 169 cm. Three digits, and all in the same unit!