Answer
An enclosure can be larger than the minimum without automatically stressing a tarantula. The practical limits are safe fall height, escape-proof ventilation, the spider being able to find prey and water, and the keeper being able to monitor a tiny animal. Floor area is usually less dangerous than excessive vertical height for a heavy terrestrial.
Why sling enclosures are different
A tiny sling in a huge enclosure can be hard to find, hard to monitor and easy to lose through ventilation gaps. Prey can hide for long periods, and a keeper may struggle to judge whether the spider has fed or moulted. A compact enclosure makes basic observation easier while the sling is small.
Adult terrestrials
For adult terrestrial species, extra horizontal space is rarely the main problem. The bigger concern is a tall gap between the substrate and lid. Heavy-bodied spiders can injure the abdomen in a fall, so use deep substrate to keep the usable height sensible.
Arboreals and fossorials
Arboreals need useful vertical structure rather than empty height. Fossorials need depth for a stable burrow rather than a huge open footprint. The right kind of space matters more than the raw litre capacity of the enclosure.
When to downsize
Downsize if a sling cannot be monitored, prey disappears beyond recovery, ventilation holes are unsafe, or the enclosure creates excessive fall distance. Do not downsize simply because somebody online quotes a single maximum size formula.
When to upgrade
Upgrade when the spider no longer has enough room to turn, maintain a retreat, reach water easily or when maintenance has become awkward. Set up the new enclosure first and move the spider with a catch cup instead of handling it.