To match the screw sizes, you can just look at the screws used in drive bay 1.
But here's a work around, if you have a hard time scrounging up screws. Take two from the caddy-to-drive connection and two from the caddy-to-chassis connection in drive 1. Two screws at opposite corners is plenty adequate to keep the drives attached to their caddies, and two screws will keep the caddy perfectly secure in the drive bay. Even if you want to order more screws, this will let you get your new SSD installed in the meantime.