From Xojo Documentation

Property (As Integer )
aWebGraphics.PenSize = newIntegerValue
IntegerValue = aWebGraphics.PenSize

New in 2020r1

Supported for all project types and targets.

The drawing pen is always square, so this specifies both the width/height (in pixels) used when drawing lines, ovals and rectangles.

Sample Code

This example sets the PenSize to 5 pixels to draw a rectangle with a thick border:

g.PenSize = 5
g.DrawRectangle(150, 10, 100, 100)