What size wire/cable is recommended to use in pvc conduit from a 50 kw generator with a run of 450'? Any help will be appreciated. Thanks.

What voltage?

For 240v with voltage drop limited to 2.95% you will want three (copper) 600MCM conductors and a 3/0 ground
(and a tugger)

1 conductors per phase utilizing a #600 Copper conductor will limit the voltage drop to 2.95% or less when supplying 208.0 amps for 450 feet on a 240 volt system.
For Engineering Information Only:
420.0 Amps Rated ampacity of selected conductor
0.023 Ohms Resistance (Ohms per 1000 feet)
0.039 Ohms Reactance (Ohms per 1000 feet)
7.199999999999999 volts maximum allowable voltage drop at 3%
7.058. Actual voltage drop loss at 2.95% for the circuit
0.9 Power Factor

7.199999999999999 volts maximum allowable voltage drop at 3%There is no 'allowable' voltage drop. Countering voltage drop is a suggestion under the NEC, not a requirement.

In addition... what type of load or loads are intended to be supplied?
The amount of voltage drop is dependent on the amount of load. Also if
This is a motor load or loads you must be careful of the inrush. This will limit the
Amount of load you can connect to the generator...