how to create custom marker

[Leonardo]

Hello,

When a marker is used in a plot, the marker does not change its size (size 
compared to screen). Two questions:

1) is it possible to create a custom marker to be used in a plot? If yes, how?

2) let's say that I have a 2D-filled region, but if I zoom in/out, the region 
will be zoomed too. How could I keep the size of the region fixed compared to 
screen (like a maker that does not change its size)?

Thanks