Update default sample size

[timmens]

In this PR, we update the default sample size.

More specifically, in the serial, non-noisy case, we change the default sample size used for linear models from p + 1 to p + 2 (where p = len(x)).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b0faac1) 78.67% compared to head (1fbea80) 78.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
+ Coverage   78.67%   78.71%   +0.03%     
==========================================
  Files          68       68              
  Lines        4953     4957       +4     
==========================================
+ Hits         3897     3902       +5     
+ Misses       1056     1055       -1     

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