I have a lot of big, mature cedar trees on my property that i'd like to make a fence out of. I understand how to split the tree in order to get roughly 4"x6" pieces of wood that are slightly triangular in shape, but i want a fence that is a fairly consistent shape and size. What is the best way to trim down those split pieces to get them looking symmetrical? Thanks in advance for your help

A saw mill is the typical way. Look for a sawyer with a portable mill, that can be set up on your property for the day. Otherwise, it depends on what tools you have available. You could split, then clean up with a power plane or hand plane if you have lots of extra time. Dress two edges square on a joiner, then bandsaw the other two edges. If you're splitting with a froe and a helper, then you can get reasonably consistent posts by marking the split, and following the marks. It's not a sawn look, but the froe offers some amount of control.

I know it might be a bit late for this suggestion but another possibility is an Alaskan mill or chainsaw milling.
Cedar splits very nicely though, so I would personally go with a maul and some wedges.

Good luck!

Split them to near size then use a draw knife and shaving buck, shave them to the shape you want. You can even go so far as to make them round that way.
Alternately you could use an adze or axe to shape the split pieces to your desired shape.