SSD for Samsung laptop in optical drive?

Your hdd should not be slowing down because it is the same connection as the hdd bay , just like in a desk top you plug both the hdd and dvd drive into the sata ports. When you say what hdd size do you get are you refering to getting another hdd or are you talking about the SSD. Most SSD's are the 2.5 size form factor and a few are the 3.5 size that are built for the internal 3.5 bays of a desk top.
Since the hdd caddy works with the 2.5 size form factor then SSD's will fit in it and laptop hdd's are 2.5 size also.

Thank you I really appreciate it!