Having new 100amp meter installed professionally,but I am burying wire to the building which is 125' from where meter is mounted. Will I have to upsize the size of wire because of the distance? I also intend to enclose in conduit, so I'm not worried about using "U" coded wire

For a 100 amp service at this length I would use #2 copper or #1 aluminum.
If using continuous conduit, you can use THWN or XHHW insulation.

2*k*i*l/cmil that is the formula for Voltage Drop

2 Being the common
K Being the type wire you are using (12.9 Copper 22.4 AL)
I For ampacity (How many amps to deliver)
L Length of the wire being pulled ( be sure to measure the route of the wire)
Cmil is the size of the wire being used located in the back of the NEC in Annex C