Brad, "in general" the panels on the lock side and opposite (sideplate) side are the same width/length/height and are parallel.
But, in some cases these panels may be wider (if the "thickness" was measured from one side across the barrel to the other side) at the front (ahead of the breech) in comparison to back near the wrist.
Also, on a number of original rifles, the "thickness" of the sideplate panel is somewhat "less" than the lock side.
What I mean is, your lock panel may need to be .250" (deep) to accommodate the lockplate/bolster etc - once the lock is snugged up against the barrel and have the wood come flush with the either the bevel or outer edge of the lock plate.
In that case the opposite panel, side plate side, may be as thin a .180" - it need not be "equal" to the lock side.
But that is dependent on what you are building - are you following a specific rifle, builder etc.
I build some cast-off in to each of my stocks - so the butt stock is on an angle from the butt plate to the nose of the comb.
I just find that if I make the side plate panel somewhat "thinner" than the lock side that it doesn't look like a "lump" sticking out the side immediately after the point where the stock straightens out again to follow the bore of the barrel.
Plus, having a thinner panel on the side plate side allows a smoother (curve) immediately behind the upper oblique flat of the barrel (between that flat and the tang).
If your entire plate is not beveled on the edge, or there is some different thicknesses, I would make it flush to the bevel (or extreme outer edge) and then leave everything else "proud" - it will look fine.