Human Interface Guidelines – size of custom tab bar icons?

I understand that I must provide icon sizes at 1x, 2x, and 3x for different screen resolutions:

A standard resolution image has a scale factor of 1.0 and is referred
to as an @1x image. High resolution images have a scale factor of 2.0
or 3.0 and are referred to as @2x and @3x images. Suppose you have a
standard resolution @1x image that’s 100px × 100px, for example. The
@2x version of this image would be 200px × 200px. The @3x version
would be 300px × 300px.

What size should a custom tab bar icon be? I can’t seem to find it in the guide.

According to the Apple Human Interface Guidelines:

Tab bar icon size:

75px × 75px (25pt × 25pt @3x),
50px × 50px (25pt × 25pt @2x),
25px × 25px (25pt × 25pt @1x)

144px × 96px (48pt × 32pt @3x),
96px × 64px (48pt × 32pt @2x),
48px × 32px (48pt × 32pt @1x)

You can check it on the link below :-