Define size unconditionally

[goerz]

All operators must implement size. This is not part of the "matrix interface"

This is a followup / correction to #18

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.9%. Comparing base (afb8c4e) to head (32f38b1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #23     +/-   ##
========================================
- Coverage    88.2%   87.9%   -0.2%     
========================================
  Files           6       6             
  Lines         303     297      -6     
========================================
- Hits          267     261      -6     
  Misses         36      36             

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