Watermarks are a great way to prevent your photos from being stolen! I personally prefer them not to be too big.

Watermarks ... only this simple questions:
Did you every see a picture in a real contest, like 'Prix de la Photographie Paris', 'Siena Photo Awards', 'Fine Art Photography Awards ', etc. which 'wear' a watermark?
Did you ever see a print at the gallery which has a watermark?

And look from the other side, if someone steel your image isn't that a sign that this image has some quality, didn't this make you proud?  Nobody steel low quality!

What do you loose when your digital images are stoolen? Nothing real! But you can win, because your picture become more popular ;)

Watermarks are often use as a way of advertisement (sometimes it looks like it is a competition of who make the best watermark) ;) 
Watermarks most of the time make pictures ugly because they often catch the eye more than the picture itself.

I know, during the last ~ 25 years several people use my images. I happy with that as long as 'users' follow the license. 

I do it this way: '*Masterpieces*' that go galleries or exhibitions for selling I don't publish online.

Its ugly though and u can always crop it out.. unless it all oveer the image.