Custom icons on macOS

Is there any way to customize the look or size of the icon on macOS? It is large compared to other icons on my dock. I imagine this is to be consistent with other Steinberg app icons. I am one of those weird people who likes my dock to look pleasing to the eye. I even have post-Yosemite style replacements for Sibelius and Pro Tools that have a cleaner look and high contrast gradient so that they look good alongside the likes of the Apple Mail and Safari icons.

Any icon in the Dock is limited to a square size: however, many app don’t use the whole square, or they use a shape of different proportions inside that square. Adobe’s apps, for example, use the whole square and will be the same size. No icon will be bigger than that square.

If you want, you can paste an icon of your choosing into the app’s icon in the Get Info window. You’ll need to restart the Dock.
You will probably have to re-do this every time Dorico updates.

Any place I might go to find a smaller Dorico icon?

Make one to your liking using your image editor of choice!