The default size can be altered in Library > Paragraph Styles > Repeat Marker Sections, but doing so will also affect any/all other characters/symbols included in that category.
Another way of doing it is in Engraving Options > Repeat Markers > Design > Coda and segno appearance > Scale factor for segno symbols. This looks like it changes the Segno symbol wherever it occurs (eg if the segno symbol is used instead of the S in D.S. al Coda). If you only want to change it in one location (eg only where the D.S. goes back to), using Custom scale might be your only option.
