As far as I know. Coinbase doesn't have some blanket minimum for everything you can trade. It kinda varies based on the actual currencies you're exchanging.
The best way to check is to log in to your Coinbase account and navigate to the trade section. Select the trading pair you're interested in, and the platform will display a minimum order value requirement before you try to place a trade.
Actually there is, the link that's been given by bitmaxz tells that most of the pairs have got $1 notional minimum limit. Some are in the form of crypto depending on the pairs but it looks like they're valued at the same amount.The best way to check is to log in to your Coinbase account and navigate to the trade section. Select the trading pair you're interested in, and the platform will display a minimum order value requirement before you try to place a trade.
For small orders, I find instant exchanges to be cheaper mainly because there is typically no inflated withdrawal fees unless your funds is already on coinbase and/or you don't plan on withdrawing any time soon.
I agree, there's no need to do the hassle of going on with registration and even KYC. You just trade there and get your desired crypto within few minutes.and what are the differences of using coinbase regards other exchanges to do spot trading?
I don't use them but I guess that it's just all about the experience and the themes and features of each exchange and as well as the limits and fees. 
