There is no minimum trade size to use the protocol. What you are referring to is an open offer to buy DAI using fiat. Anyone can create an open offer of any trade amount. The mail method on that offer means the desire to send fiat by mail in exchange for DAI.
Ok, that means that using fiat > DAI, there's a minimum amount for purchase. Whilst, using crypto to crypto transaction, there's none? I think I've understood it, thank you for that.
Not really, I mean there is one offer there right now where the person is willing to buy between 50-100 DAI with fiat. Other people can create other offers with different bid amounts.