How do you size the holes so pixels actually snap in?

This is almost certainly a combination of slicer calibration and printer tolerance. Even when a CAD model calls for a perfect circle, the physical extrusion process always introduces slight variances. If you are using a basic Ender setup, and your walls are over-extruded, that excess material will be pushing against the inner edges of those small holes, making them far tighter than the designer intended. Instead of scaling the entire model down a couple percent, which works but is crude, I recommend opening the STL in your slicer and adjusting the hole size parameter specifically. Look for an extrusion or diameter offset setting if your slicer allows for it, or if you are comfortable moving to the original design software, just bump the inner diameter by 0.2mm or 0.3mm. That small increase should provide the necessery wiggle room without making the holes look disproportionately large. Sometimes increasing the layer height slightly also helps with rounding off sharp corners, which can prevent the pixels from binding.

Regarding material, PETG is definitely the better choice over PLA for outdoor seasonal use. PLA is brittle in cold weather, and while it works fine indoors, its rigidity makes it prone to snapping when you're seating or pulling the pixels. PETG offers much better impact resistance, which is critical for props that are going to be handled outside. You are right to consider the summer heat, though; PETG has a lower glass transition temperature than some engineering plastics, so if these props are going to sit directly under intense sun in a very hot attic, they could deform. For that specific scenario, maybe consider ABS or ASA, but those are much harder to print reliably on an Ender style machine.

For general garden use, PETG is robust enough for the winter and should handle the occasional heat blast from the summer without becoming overly soft. Just make sure your print temperature profile is dialed in perfectly for the PETG to get those corners right. Good luck with the snow props!

Kind of a side question since my setup is diffrent, I mostly do mini trees and window frames so I never seat pixels in printed plastic, I zip tie everything. But for a hundred bulb string I zip tied to a wooden frame one year, I drilled the holes slightly undersized and opened them up by hand until the bulbs clicked in snug. Would that same trick work here, or do the printed tabs snap too easy to ream them out? Also on the storage thing, I keep mine in a plastic tote in the garage rather than the attic and they've been fine so far, though I'm in a milder climate than some of you prolly are.

I do a quick test print before committing to a full run. Just a little strip with the same hole printed at a couple different sizes, takes ten minutes and saves you a whole sheet of wasted snowflakes. I've also had decent luck just reaming the holes with a drill bit by hand, same idea as the zip tie guy above. Goes slow and you won't snap a tab. On material, I've left PETG props in a shed for a couple summers with no warping that I could see, so I wouldn't sweat the attic too much unless it gets seriously up there.

Agree with the PETG call, my PLA mini tree stars cracked the first hard freeze two winters back, swapped everything to PETG after that and haven't lost one since. One thing I'd add is that hole size seems to shift a little depending on whose pixels you're using, the ones from my second string order sat way looser than the first batch even in the same printed holes. So once you find a size that clicks in nice, don't assume it carries over to a different batch, do one test seat before running the whole stack.

A cheap set of digital calipers fixed this exact headache for me last year. I was guessing at hole sizes til I actually measured a pixel from my own strings, and my holes were off by enough to matter. Measured the bulb, measured the print, adjusted, done. Also second guessing the designer defaults, mine never seemed sized for the pixels I actually own. On the material side I print mine in PETG stored in the garage all summer with no sagging, but mine sit in the shade, so a hot attic might be a different story.

Update: tried the test strip idea with a few hole sizes on one little corner piece and it worked great, found my size in one go. Also swapped to PETG like you all said. One thing though, one of the replies said hole fit shifts between pixel vendors. My strings came from two different orders, so do you all sort them by brand before seating, or just test each batch separately?