Just out of curiosity, I inspected the yarns database (the search engine//cralwer) to find the average length of a Twtxt URI:

$ inspect-db yarns.db | jq -r '.Value.URL' | awk '{ total += length; count++ } END { if (count > 0) print total / count }'
40.3387

Given an RFC3339 UTC timestamp has a length of 20 characters with seconds precision. We’re talking about Twt Subject taking up ~63 characters/bytes on average.