Rashi says explicitly each day of creation was 24 hours. See as well the Divrei Chaim (Chanuka p.45 col. 4) quoted above.
The Gemora says this expicitly. It describes 10 things that were created on the first day of creation, one of which is the “length of the day and night” – as it says, “vayehi erev vayehi voke yom echad”. So the time span of the day was created on the first day of creation. And, as Rashi states, it means “[the day and night together] – i.e. 24 hours between them”.