Hallo I found a Mod (on a other site) which let you ride bigger animals - cougars for example.

Now i wonder if this is possible, is it possible with horses too? 

Is it possible to make the horse breed "Arabian" bigger? So that they have the same size as the "Missouri Fox Trotter" breed?

You can do that with the Rampage trainer over on Nexus, I've used it to spawn in a Gypsy Cob for Arthur and sized it up to be as big as the raven black Shire, but it does break some functionalities with the horse. If you scale it up you can no longer lead it around or pat/feed/brush it unless you're in the saddle. Won't work while standing next to your horse.

In the .ymt file for the horse, up towards the top there is a line: <scale value="1.09025000"/>

Adjust the number to your liking. The number I posted is the size of a shire. A belgian is 1.07 IIRC. This will also change the size of all horses using that same .ymt file FYI.