I have a 2 horse cotner straight load with room infront of the horses breast bar, no manger and a full escape door on each side and a ramp, 7' tall and extra wide. So far no horse has refused to enter it. My boys are both 16.3+ and not skinny TB types. But have had horses refuse to enter my friends very nice 3h slant.
384596_2890299617807_1269722567_3132618_1222143478 _n by Cecillia B Photography, on Flickr
Both of our trailers are white inside. Only other issue I encountered with the 3h slant was adapting the OTTB's to the slant, they don't seem to like it at first, prefer to ride straight but all are riding in the slant fine now.
I agree with another poster about fitting the trailer to the horse. See what they like to ride in if you can.