A página de manual de script
dá a resposta:
-t
,--timing
[=file]
Output timing data to standard error, or to file when given. This data contains two fields, separated by a space. The first field indicates how much time elapsed since the previous output. The second field indicates how many characters were output this time. This information can be used to replay typescripts with realistic typing and output delays.
Então, "tamanho de cada pedaço" está correto.