Should I trim the top or the bottom of this oversized front door?

If you're looking at a standard door and trying to bring it down by two inches, the first thing to verify is the material. If it's a solid wood door, you have some room to work, but if it's fiberglass or has a hollow core, cutting two inches will likely expose the hollow center or the foam interior. Once you cut past the solid stiles and rails, the door loses its structural integrity and you'll have to find a way to glue a new block of wood back into the cavity to give it some strength. The handle height is ussually the biggest deal-breaker here. Standard knob height is 36 inches. If the hole is already bored and you take all two inches off the bottom, that handle drops to 34 inches, wich feels weirdly low every time you reach for it. On the flip side, taking two inches off the top might leave the top rail looking too thin compared to the rest of the frame. Splitting the cut—one inch off the top and one off the bottom—is almost always the better move for keeping the **proportions** balanced.

You also have to look at the hinges and the bottom sweep. If the hinge mortises are already cut into the door, trimming the top or bottom will shift their position relative to the frame. You'd likely have to plug the old holes and cut new mortises in your existing door jamb to make them line up. And since this is a front door, you’ll need to make sure the bottom is still compatible with a weather sweep. If you trim the bottom, you might lose the factory groove for the seal and have to screw on a surface-mounted sweep instead. Measure the distance from the top of the door to the center of the handle hole before doing anything. If that distance is already standard, taking an inch off both ends keeps the handle close enugh to a normal height that nobody will notice. Just make sure your brother-in-law uses a fine-tooth blade and some painters tape to prevent the wood from splintering during the cut.

So if I try to split the cut like you suggested, how do I know if there's enough solid wood on the top rail specifically? I’m worried about hitting a hollow spot if it’s one of those "engineered" wood doors that just looks like a solid slab. Also, is there a specific way to measure for the hinges if we do end up shifting everythign? I'm trying to follow along but the technical side of the mortises is a bit over my head. My BIL is handy but he tends to just wing it, and I'd rather not buy a second door if he messes this one up.

To check that top rail, rap your knuckles along the edge. If it sounds thin, it's likely veneer over a particle core. Most quality exterior doors have about two inches of solid wood up there, but you can drill a tiny pilot hole in the top edge to check the shavings if you're relly unsure. For the hinges, don't stress the math. Lay your old door directly on top of the new one after trimming. Line up the top edges and use a utility knife to score the hinge locations onto the new wood. It’s way more precise than a tape measure for getting those mortises to line up perfectly.