Ok I am assuming that you bought a streamer with an internal SSD slot... Buying one and putting it in is a waste of time because when you burn more music you have to now find a way to put it on the SSD drive that is now screwed into the slot provided by the streamer or figure out how to connect it to your computer to do it which is terrible...... you have a USB slot also I'm assuming... if so buy an extension that fits into the usb port (from ebay or Amazon) move it to around to the front where it is easily accessible... once you've done that buy a flash drive whatever amount of GB you need (flash drives are essentially SSD drives) and then put your music on there. It allows you to remove it easily and change your media around without difficulty.
There are basically three types of SSD drives which have been explained very well above... two are internal and one external and yes size does matter in this case. If you are going to use the slot provided by the streamer, it has to fit correctly and usually has a hard cover around it. The second type is open and usually are used in computers... so get the correct one.
Finally if you decide to buy the internal one (I did at first and took it out), you will realize what a hassle it is to deal with and then you probably should move to the method I use.