The splice marker glyph cannot be resize but you can create the define the new glyph for splice marker in defaultstyle.xml file in config directory and use this in the styling.
There is document available to configure the splice marker:
How to create glyphs in defaultstyle.xml file.
Please let me know if this helps you.
I also need to make the splice markers smaller (in Logic). Both of the links that amit_raj posted are coming up 404 Not Found. Are these documents available somewhere?
I am able to follow the links - have you tried copying the link into the browser?
The links work for me today. Thanks.