preview-scale-function in the
preview-appearance group controls this. From the customization menu:
Preview Scale Function: [Value Menu] preview-scale-from-face
Calculate preview scale from ‘preview-reference-face’.
This calculates the scale of EPS images from a document assumed to have a default font size given by function ‘preview-document-pt’ so that they match the reference face in height.
Scale factor for included previews.
This can be either a function to calculate the scale, or a fixed number.
Try setting the value to various fixed numbers. The preview sizes should shift up or down accordingly.