The Aventine Hellenistic horses I have - which are probably the same basic sculpts they use for the Romans - and the A&A MIR horses are a very close match in size. If you want bigger options, then - in addition to the Wargames Atlantic plastics mentioned - you could try Aventine's new Dark Ages horses, which are noticeably taller than their Hellenistic beasties or the A&A offerings.