Having just bought a Thailand super red HMPK myself...I can tell you Express shipping is nothing to the PITA and expense of importing a fish. It is my first time importing and unless it is one hell of a nice fish probably my last time.
Being a US fish it will probably get to 3-3.5", my own giant is 3.5" and large by betta standards...still a small fish. A 5.5G is fine IMO...bettas arnt notoriously active or messy.
150G SA Cichlids|100G Planted Community|50G Reef|20G Tanganyikan|10G Divided Bettas|10G Nano Fish
Common decency...imagine the nerve!