You may have to make a new partition on the SSD before you can do anything - https://technet.microsoft.com/en-us/library/gg309170.aspx
Does this guide work for windows 10 too?
Yes.
The Specify Volume Size page specifies the minimum and maximum size for the volume in megabytes and lets you size the volume within these limits. Size the partition in megabytes using the Simple Volume Size field and then click Next.
Can this be any number or is there a number i should put in here?