It depends on the size of the tank. Little vintage trailers are not built to support the weight of a large tank underneath, so the tank would need to fit on the tongue. I'd say no longer than 32" depending on the shape of the tongue and width. Motorhome tanks require very specific bracing and access. Do the research in your area, as they may not be legal on a trailer.
Horizontal tanks are very expensive. Is there a reason you want one?