When working with Spine, the character size just needs to be large enough to be comfortable: easy to see, large enough to place things precisely, etc. If it's very small or very large, it's less comfortable to work on, though this can be adjusted for with Bone scale.
At runtime, the size can be the same, larger, or smaller than in the editor. The same skeleton and animation data can be used with different size images. This means the size in the editor should never be a limiting factor, even if you have chosen a very large or small size.
Spine's UX is optimized for roughly sizes that fit on your screen at 100% zoom. Smaller or larger is also fine, but you may find some UX issues like zooming is too fast or tool slow, or the bones and other icons are too small or too large. To fix that, adjust Spine's Bone scale setting. Eg, increase it if your skeleton is very large, decrease if very small. This will solve the problem with not being able to zoom out far enough because your skeleton is large, and other UX issues.
It's common to paint at a higher resolution, scale down for work in Spine at "reasonable" sizes, then pack a texture atlas for whatever sizes are needed at runtime. Some apps pack at multiple sizes and use the closest one based on user screen resolution, improving graphics quality.
When packing a texture atlas there are no size restrictions except what your GPU can handle, but consider how much video memory will be needed to load your images at runtime and what devices you expect your app to run on. Many apps target a 2048x2048 or 4096x4096 max texture atlas page image size.
Import Data with JSON can be used to change the size of the images you use in the editor:
https://esotericsoftware.com/spine-import#Scale
