I thing purcashing evergreen styrene rod for making wielded seams.
do anyone know the right size?

How wide is a weld? Or indeed a piece of string. There is no right answer and my approach is to work from photos or (in certain circumstances, if I trust the draughtsman to have measured them accurately) from plans.

When I used styrene rod I tended to use up to 0.3mm diameter but I find that I can get more convincing welds (they are solidified molten metal after all) using Gunze Mr Dissolved Putty for smooth welds or epoxy putty for the rough ones as found on the ISU for example.

As a professional welder I can tell you that there is no "right" size for weld seams, depending on the size and the position, the width and thickness of the seam can vary a lot. Best is to search refernce pics and built after that. And don`t forget the welding technique MAG and MIG seams can be different from WIG/TIG seams.

THANKS!