I want to deliver 50 amps to my well sight using direct burial triplex aluminum. It is 625' from existing power. What size wire should I use?

Too many variables to answer right now.

What are you powering? Just the well?
Why 50A?
What does the well draw?

625' is a very long way to go with 120/240V. Is there any closer source?

Even at a 20 Amp draw, #6 Aluminum shows:
Voltage drop: 15.57
Voltage drop percentage: 6.49%
Voltage at the end: 224.43
#4Aluminum At 50 Amps is worse:
Voltage drop: 24.49
Voltage drop percentage: 10.20%
Voltage at the end: 215.51
Closest to 5% loss is 1/0 AWG
Why is the well so far away? 625 ft. is not going to be easy without a pole and transformer.