In the photos it looks like it may be a small sporran intended for a youth or child. What are its measurements?

A regimental sporran will have a cantle/rim 6 to 7 inches across, and a length of around 18 inches.

Anyhow, that St Andrews badge is a Black Watch style badge, but that's not a Black Watch sporran, nor indeed the sporran of any of the Regular Highland regiments.

I think it's simply a civilian sporran. It's quite common for old civilian sporrans to have those St Andrews badges on them, also thistle badges, or Rampant Lion badges.