I think the confusion here is in the terms 'UV' vs 'template' vs 'texture.
where:
UV is the coded map of the object as it presents itself to the render engine
Template is the 'template' of the UV mapping, used in graphic apps to paint on
Texture is the painted template of the uv mapped object.

Now, in UVMapper one is asked "What size to save out the TEMPLATE" and usually has a default of 520x520 (?) but can be changed to 'whatever' and not even need be square.
The question here is again:
IF I saved out the TEMPLATE at default 520x520 but then in Photoshop enlarged it to 4000x4000 and painted a Texture on it. Save the Texture at the 4000x4000
One can paint much more detail on a 4000x sheet of paper then on a 520x - but both being applied onto the SAME Obj with the same original UV mapping.

I think FVerbaas and ironsoul answered the question.
Also the T-shirt I was speaking about does render out much better with the larger Texture then had I painted onto the original smaller map.
No "stretching" involved....sorry, I did not understand that explanation at all but have seen ill effects if say the Original map was Not square (say 1024x762) and I RESIZE the TEMPLATE (or texture) into a 1024x1024 and paint on it....IF I then apply that texture to the object it 'stretches' the texture UNLESS I run the object file back through the mapper and save out a 'squared' UV map (ie Template) and replace the original model with the newly mapped one.....

Anyway: resizing the TEMPLATE did allow me the results I was looking for - thanks all