The author wrote that he currently has ~ 63GB occupied on C. From this it can be concluded that 120GB is enough.
However, if you play a lot and you have enough money, buy a larger SSD right away because everything loads faster.
I purchased SSD for a second computer this month, but I do not play at all, and everything on my C after 3 years after installing the system takes 54GB, so I bought 128GB and I'm happy.
Nobody will make that decision for you. You have to decide for yourself what is most important to you. If the games will be installed on the HDD then the remaining money can be used for other purposes