The size of the thumbnails you would use depends on the type of site you have. As long as the thumbnail is clear enough so people know what it is an image of then it serves its purpose. It need not necessarily be a reduced version of the larger image, sometimes showing a cropped section of the larger image can work better. Most of the thumbnail images I have ever created have been around 1k in file size.
In my latest project, I had thumbnails sized to 132*100. I wanted the width to be greater than the height and it perfectly fitted the design. In other words, there is no one single right size. It all depends on your requirement obviously.