[LIST] Features list for devices is a good list for VoLTE support in the US per carrier. I think the smallest listed known to work with T-Mobile among those are
- Xperia XZ1 Compact with 4.6" on 129 x 65 x 9.3 mm (5.08 x 2.56 x 0.37 in), weight at 140 g (4.94 oz)
- Pixel 4a with 5.8" on 144 x 69.4 x 8.2 mm (5.67 x 2.73 x 0.32 in), weight at 143 g (5.04 oz)
commercially available
- Teracube 2e via the eshop is already at 6.1" with (HxWxD) 155.2 × 73.3 × 10.1mm, so I think it doesn’t qualify