Personally I’d go for as big a UPS as I could afford, but I serve some public-facing stuff from my homelab and I live in an area with outdated infrastructure and occasional ice storms. I currently have a small UPS and have been too tired/overwhelmed to set up automated shutdown yet. It’s not too hard though, I’ve done it before. And even without that in place, my small UPS has kept things going thru a bunch of <10 minute outages.
Check out ServeTheHome’s “Project TinyMiniMicro” on Youtube for a great overview of ultra-small form factor (“1 liter”) business PCs.
The big three PC makers each have standardized products in this form factor with (relatively speaking, compared to smaller manufacturers) tons of spare parts available.