I'm not sure how the Dub libraries are set up but I thing that if the horse installed correctly, you should be able to open the Character folder in your Poser library and under that, there should be a Daz Animals category where the horse should show up. 

I kind of doubt the sizes are the problem but if you Right click once  on a file of any type and select Properties from the menu that pops up, you should get a popup dialog that shows the information.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken