As far as I can find, pies only support 32GB cards, or 64GB with a trick. I'm using a 128GB card and it worked on the first try without any problems, so what am I misunderstanding? Is the information I'm finding out of date, or am I asking for some sort of trouble down the line. I have filled them up all the way with no problems so far. One thing that might be different about my setup is that I cloned the image off of a 32GB pi. I am just using the regular partition scheme if that matters.
First, your premise is incorrect; there is no 32/64 GB limit imposed by the RPi OS. Therefore, I'll re-phrase your question as asking, "What is the size limit for the RPi microSD card?"
The question of size limits for storage media is an old one, and a complete answer would be quite complex. If you're interested, this 3-part article covers storage media limits from roughly the commencement of the era of personal computers through 2002. And this Q&A on U&L SE covers some of the considerations from a Linux perspective - as of ca. mid-2012.
If you quickly scan the references above, you may form two hypotheses:
There are numerous factors that determine storage media limits, and
Storage media limits are often not the total media capacity, but rather it is the partition size that is the limit.
From the previous answers and comments to your question, it seems that "The Foundation" has not addressed this subject in their "Official Documentation" for the SD card. This should not be surprising, as there are numerous gaps in their documentation for (probably) several reasons. However, this could also be taken as a reflection of a situation in which there are no microSD cards manufactured today that are too large for the Raspberry Pi OS. I suspect this is the case, and I will propose this is the answer to your question.
You may also consider the following:
The SD card for a Raspberry Pi contains (at least) two partitions. One is a
FAT32partition - with a 2TB limit on its partition size, the other an
ext4partition - with a 1 EB (1 million TB) limit.
Up to this point, we've considered the question of microSD card capacity limits only from the perspective of the RPi OS and file system partition limits. However, there are also limits inherent to the design of the Secure Digital (SD) format. Referring to the table at the end of this article, we find that for a microSDXC Type card, the limit is given as 1TB; for a microSDHC Type card, the limit is given as only 32GB. It has been reported that microSDXC Type cards work fine in RPi, and so this is one source for vetting your microSD card prior to purchase.
pies only support 32GB cards
I don't think there are any official statements about this, and YMMV. I've been using 64 GB cards for years, at least on 3/4 models, and never had a problem.
It did not take me more than 2 minutes internet searching to find multiple claims that 128 GB cards and even bigger should work. Eg, someone here claims "up to 128GB is tested and confirmed / guaranteed" (although they provide no reference...).
I can't think of many reasons why any particular large size SD card would not work. If there were any problems with that configuration specific to size, it should fail immediately with a specific error, not mysteriously later after it worked.
The only possible size limitation would be filesystem related, and the only filesystem with size limitations that would affect an SD card right now would be the vfat32 filesystem, which typically would not be used with a raspberry pi.