Is there a way to have an item sized correctly or at least very close to the item you are appending it to as a subtool?
“sized correctly” is a relative term. If you’re building geo in an outside application and things are scaled relative to one another then imported/appended geo will be sized correctly.
If you’re just adding a sphere for an eyeball to a whole character, then no. Zbrush doesn’t know that is going to be an eye, or a head, or a dragon.
These are meshes made within zbrush.
If you are making the meshes in ZBrush then you can use the Floor grid (you can turn it on for other axes) as a guide. Or for more precise control, use the units & ticks on the Transpose ActionLine (you can adjust these in Prefernces>Transpose Units). Also you can build your new mesh (for example using ZSpheres) while already appended as a subtool, therefore it will be the right size from the start.