It is easy to unlock the size of datum planes but yes, it is all about preference. -If- I used start parts, I would certainly lock the size to my normal expected sizes. Pretty much everything I work with fits in a shoebox. A big shoebox maybe, but certainly not a diagonal of 881.0254 inches.
I don't know what has changed since my last heavy involvement before Creo was 2000i with a brief stent with WF4. However, the relative accuracy is dependent on the current bounding box. It seems to be dynamic where only once did I have a problem with creating a first feature where the relative accuracy caused the feature creation to fail. This is the exception rather than the rule.
I find myself resizing datum planes and axes on a regular basis. All too often I have a plane stick out into no-where simply because a sketch feature exists out there.
I would much rather have a config.pro setting with initial_boundary_box 1,1,1 options. But that's not the case today.