Where will the meter be mounted? The power company selects the conductors and materials that supply the power up to the meter. In my area, the power company provides and installs those conductors. Any problem or power loss prior to the meter is their problem for them to solve. If the meter is planned to be on the pole and then 300 feet to the house, not so good. I want my meter closer to the house.

In this neck of the woods, the POCO
will not allow the meter to be any
more than 200 feet from their pole,
(they don't want to be responsible
for any voltage drop at the meter).
If you're against using conduit, (NOT a good
idea), then at least pad your trench
with sand and use a marker tape a
few inches above the buried wire.
The minimum size wire for a 200
amp service, (around here) is 4/0
Aluminum, but because of the distance
you might want to go up a size.
You will have to contact the POCO
eventually, so give them a call now
and find out all of their requirements.