I got the ota update for kitkat and after that installed the android launcher ripped from nexus 5 rom. The result is that my icons look too big. Is there any way to decrease that or use a custom rom to change the size?
Unless you write you own Homescreen launcher or use an existing one from Goolge Play, there's no way to resize icons.
Well, no way does not mean its impossible:
Use another launcher (for example, Nova Launcher).