If you want something smaller, xperia z3 compact. No WiFi calling though or band 12
Frankly, I'd wait... There is a rumor that WiFi calling will be integrated into android 5.0 itself next year , hwhich would make the z3 compqct worth getting
Otherwise, the only other T-Mobile phones are the z/z1s which are 5" , slightly smaller than z3's 5.2". Z/z1s have WiFi calling buut no band 12 either