Silvia had such a long face
Isn't 'cara de caballo' the equivalent of someone having a long face, as in 'Why the long face Joe?' Not literally a long face, but a look of sadness?
I don't often hear people comparing others faces to a horse's in English but I have heard on occasion that someone had a 'horsey-face' although this is very colloquial. If you really are comparing her face to a horse's, I would say 'Silvia has a face like a horse'.
I think if the text that comes before or after your sentence talks of Silvia's mood or something of the like, you shouldn't say she has a horse like face. In that context it wouldn't make sense and would be an insult!
native English speaker
Local time: 23:52
Native speaker of: English
PRO pts in pair: 66