You would have to open the image up in paint and resize it there.
JNealand had a post on some great tools for vixen, it was called "must have tools for Vixen" here is a clip from it
Editable Adjustable Preview - lets you change your preview around and easily lets you use predefined images for mini trees, snowflake, mega trees, etc. Has ability to put RGB pixels in place and map to your channels etc.
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