How can I reduce the size of a PNG in unity while maintaining image quality?

I am new to Unity. I use Unity 2019.1.3f1 staff.

My problem is that I have created images (PNG) of a character in Photoshop. The margins look clean in Photoshop and look fine when pasted into Unity, not the best. The original image is too big, but when I reduce it, the image quality suffers.

I understand that I can downsize the image in Photoshop, but is that the most efficient way? If not, how can I resize images in Unity and maintain image quality?

Below is a comparison of the image side by side after I resized (left) and before resizing (right). Suggestions is much appreciated; Thanks for your time,

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