Bone scale is the right setting to increase the size of bones in the viewport, without making the UI huge. Spine is optimized for a particular size, making the size of bone icons, zooming, and other features more comfortable. If your images are much smaller or larger than typical, changing the Bone scale increases or decreases that ideal size. Typical sizes are a range, roughly the sizes used in the example projects.
The bone icons shouldn't become excessively blurry from changing Bone scale, at least not in more recent versions (eg 4.2+).
Note if you are using massive images, it will work fine, but there's not much benefit to working with huge images in Spine. You'll need to adjust Bone scale and you may see a green loading placeholder flash, as it takes longer to load huge images. You can work in Spine with different size images than you use at runtime, if you need small or large images at runtime for reasons.
