You can place block inserts in model space and specify the size that they should appear to be in paper space. If the zoom factor changes, you can update previously set objects to adjust their size accordingly.
Select objects: Select the block insert objects
Update or set paper space scale for blocks [Set/Update] <Set>: Choose to 'set' or 'update' paper space size
X scale factor relative to paper space or [XYZ] <1.0000>: Specify the scale values
Y scale factor relative to paper space <use X scale factor>:
See also: BSCALE