Here are a few comparison shots between Relic, Perry's, WargamesFactory, Warlord Games and Immortal Miniatures. Some are slightly converted so read the descriptions please.
1. Relic Miniatures- Thracian
2. Perry's - Sudanese Tribesmen with ImmortalMin. hat (psiloi/peltast)
3. Wargames Factory - Hoplite
4. Wargames Factory - Hoplite with WF Persian head (Takabara/Kardakes)
5. Wargames Factory - Persians
6. Warlord Games - Macedonian Phalangite
7. Immortal Miniatures/Warlord's - Hoplite
8. Warlord Games- Celtic Warrior with ImmortalMin. helmet (Galatian)
9. Warlord Games- Roman Imperial Legionary (scorpion crew)
Hopefully that gives you some ideas as to sizes. The bottom image shows Perry's Sudanese conversions to psiloi. The one above shows that if you want to have some sort of Parthian/ish opponents for Warlord's Imperial Romans... don't, look at the size difference!