At events like these, most people seem delighted by anything unique or creative, so go with your intuition. If they criticize, don't invite them back. Just kidding! I love larger cookies myself, because then I get twice as much cookie to eat without having to raid the cookie tray a second time, lol. Are you stamping the unbaked cookie or fondant for an impression, or using edible colors to "ink" the design on?
If you have to go smaller, you could have just the horse's head peeking onto the edge of the cookie, playing peek-a-boo. Also, you could consider cropping off just the rear of the horse, or make a few stunning large ones for the wow factor on a mixed platter. Put out lots of mini cookies in matching colors or with little motifs stolen from the horse design. I am sure lots people will post great ideas for you here, so hang in there.
P.S. If you don't already know, you may want to adjust your oven temp./time, so the whole cookie is baked through before the edges get overly crispy.