What's the maximum amount a bullet can be sized down?

The driving bands will.most likely be your limit. There are other problems like bullet deformation and work softening. More than a few thousandths is abnormal and may cause problems.

.401 to .375 is going to be waaaay too much... as is .357 to .375...

DM

Not going to work satisfactory. I've sized .008 before and did it in two steps to keep from mangling them and having uneven driving bands.

I've heard of folks sizing 358 jacketed bullets to 348 for that cartridge.

I sized a bunch of .452 down to .430 once in an emergency. It was in a Starr "push through" sizing press. Not the best idea I ever had but shot a small doe with one of them.

If you have a sturdy compound press like a rock chucker adjusted near the end of the stroke and lube well you might be able to get by with 3 or 4 steps depending on how hard the alloy is and how many and how deep the bands are. .026 is a lot. It won’t just squeeze the bands in, it will smear them downwards as well and as they get smaller it will size much harder so the last .007 will be much harder than the first. Hard alloy and 1 or 2 lube grooves I think it’s likely a no go past .006 or .012. Putting a 357 in a 375 isn’t likely to work well either, it won’t end up straight in the bore and I think may lead like crazy as the powder gasses melt the outside of the bullet. Seat and crimp could be difficult as well

Most I've ever went is .020 sizing bullets ment for c&b revolvers down for 44 mag light loads. Funny enough I'm actually making some right now for a forum member. There will be a skirt on the base I remove then powder coat.
These are .450 and will be .430