There is no function to get their real ingame size or hitbox. You can only set one of it using CBaseModelEntity:SetSize(mins, maxs).
Please add a good API for working with entity model's hitboxes. Something like:
CBaseModelEntity:GetHitboxBounds(pszHitboxName)
CBaseModelEntity:SetHitboxBounds(pszHitboxName, vMinBounds, vMaxBounds)