This is a surprisingly ridiculously complicated question and depends on a large number of factors.
Is this default meant to be in this document or in all future documents?
Do you intend to change existing lines or just new ones?
Are you using shapes from stencils to drag lines onto the page or just using the new line from the Home>Tools dropdown button?
Are you controlling the look in the docs using themes?
I don't have time to go through the whole gamut of the nuances so I'm going to assume that you are just talking the current doc and using the new line button on the home tab.
The look of those lines is controlled by the shape attributes of a style called Normal. In the recent versions of Visio, editing and applying the styles is buried and not easy to find. You should add two commands to your Quick Access Toolbar (the buttons that sit in the top band of the window). You do this by clicking on the odd down arrow button at the end of the QAT - hovering over the button should show 'Customize Quick Access Toolbar'. Click it and choose 'More Commands' and add two buttons [Define Styles... and Style>]. Once those buttons are on your QAT, you select one of the lines and you will see the style applied to it in the QAT. You click on the green circle button to Define Style and Change the shape attributes to match your preferred line thickness.
Assuming the other lines already on the page also use this style, they will all change and new lines will also be using that style.
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Andrew Lockton
Chrysalis Design, Melbourne Australia
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