Is Ratchet & Clank: Size Matters on Steam?

No. Ratchet & Clank: Size Matters is not distributed on Steam, so there is no Steam Charts or SteamDB player count for it. Its live popularity is tracked here via Twitch viewership instead.

What platforms is Ratchet & Clank: Size Matters on?

Ratchet & Clank: Size Matters is available on PlayStation Portable, PlayStation 2.

When did Ratchet & Clank: Size Matters release?

Ratchet & Clank: Size Matters released on February 13, 2007.

Ratchet & Clank: Size Matters

Feb 13, 2007ShooterPlatformAdventurePlayStation Portable PlayStation 2

Ratchet & Clank: Size Matters — frequently asked questions

Is Ratchet & Clank: Size Matters on Steam?: No. Ratchet & Clank: Size Matters is not distributed on Steam, so there is no Steam Charts or SteamDB player count for it. Its live popularity is tracked here via Twitch viewership instead.
What platforms is Ratchet & Clank: Size Matters on?: Ratchet & Clank: Size Matters is available on PlayStation Portable, PlayStation 2.
When did Ratchet & Clank: Size Matters release?: Ratchet & Clank: Size Matters released on February 13, 2007.

About Ratchet & Clank: Size Matters

Ratchet & Clank: Size Matters (known as Ratchet & Clank 5 in Japan) is a 2007 platformer video game developed by High Impact Games and published by Sony Computer Entertainment for the PlayStation Portable and PlayStation 2. The game is the first Ratchet & Clank title on the PSP handheld. Development company High Impact Games was spawned from the original Ratchet & Clank developer, Insomniac Games. The story is about Ratchet and Clank as they are interrupted from their vacation to search for a kidnapped girl and encounter a forgotten race known as the Technomites.

Summary via IGDB.

Ratchet & Clank: Size Matters on Wikipedia

Ratchet & Clank: Size Matters is a 2007 third-person shooter platform video game developed by High Impact Games and published by Sony Computer Entertainment for the PlayStation Portable. A spin-off of the Ratchet & Clank series, it is the first game in the series to be released for a handheld console. Developer High Impact Games consisted of former employees of series creator Insomniac Games. A port to the PlayStation 2 was released the following year.

Source: Wikipedia (text under CC BY-SA 4.0).