FWIW, in addition to all the other facilities it provides to photographers (eg, great printing, slideshow, cataloging, ACR adjustments, preset management, etc.), Lightroom absolutely excels at quickly producing watermarks on tons of images of different sizes, and allows lots of ways to very, very quickly customize the watermark's size (proportional or fixed), placement, orientation of the watermark, as well as whether it's text based or image based, color, etc., etc..
If one is sending out hundreds of photos from an event, the absurdly low price of $150 (or USD $80 for the student/teacher edition) means that LR pays for itself in a couple of hours of saved labor costs, and it's pure profit after that. If one is dealing with small numbers of images, maybe even just one carefully crafted image at a time, then the LR makes less sense, but it sounds like the OP may be in the same boat as me in terms of dealing with larger numbers of images.
HTH,
Tom M
PS - Here are a couple of screen shots that show some of the flexibility in just the watermarking part of LR as well as an example of a fixed-percentage size watermark produced by LR.
