From FCKeditor Docs

You can set thumbnails parameters byc changing the values of the parameters:

Set Thumbnails = server.CreateObject("Scripting.Dictionary")
Thumbnails.Add "url", baseUrl & "_thumbs"
Thumbnails.Add "directory", baseDir & "_thumbs"
Thumbnails.Add "enabled", true
Thumbnails.Add "maxWidth", 100
Thumbnails.Add "maxHeight", 100
Thumbnails.Add "quality", 80

• maxWidth and maxHeight determines the maximum size of the thumbnails which can appear in CKFinder. These values are expressed in pixels. Of course you can increase them but than the thumbnails will look more like the full-sized images rather than miniatures.

• quality sets the quality of the thumbnail in a range from 1 to 100. The smaller the quality the smaller the size of the thumbnails. Notice that an acceptable value of the quality is about 45 and the mostly used is about 80.

• You may disable the thumbnails by placing the bool value false in the enabled section.