There’s two things here. First, there is the grid made up of visible lines. That’s limited in size - and limited in usefulness anyway. Second, there is the grid you can snap to. That extends across the entire canvas, not just where the visible grid is. Way bigger than your car.
In any event, once you introduce lines at angles and curves you end up with points that often don’t lie on the grid and the grid just becomes a nuisance.
There is absolutely no reason not to modify the supplied template files or to create a new template file.
A template file is just a standard .3dm file, nothing more. The template files supplied with Rhino are simply examples. There is nothing special about the files or their names. Modify, delete or change the names as desired.
Using New and selecting a template file is exactly the same as using Open and selecting the template file with one exception. When New is used rather than open the path and name of the template file is not displayed at the top of the Rhino window nor is it used by the Save or SaveAs commands. That keeps the template file from being accidentally modified. Any Rhino .3dm file can be used as a template file.
You can create a template with millimeters as the units and a grid which a 300 meter ship fits in with any desired spacing of grid lines (multiples of mm’s only). Attached is an example with millimeters as the unit, minor grid lines space 1 cm apart major grid lines space 1 m apart. 300m millimeter.3dm (49.9 KB)
Files used as templates can include geometry, and the geometry will be present when New is used and the template with the geometry is selected. This can be useful if the same geometry, for example construction lines or planes, is used repetitively for a particular type of model.
I think the history behind it is that back in the V2 or earlier days, there was no ‘millimeter small’ or ‘millimeter large’ template, just ‘millimeter’. The default file tolerance was 0.01. People complained that that tolerance was too large for smaller objects, so the answer was to create ‘small’ and ‘large’ templates - the only difference was simply that the abs tolerance was lowered to 0.001 for ‘small’.
So much for the history, I am actually in agreement here, perhaps the ‘large’ template could have a grid that is an order of magnitude larger…