Not very big. 20kt is in that funny "operational" range where it is too big be a tacnuke, and realistically too small be to really be a strategic yield by today's standards. It's a nice city buster, but not "big". "Big" means "mega".

As far as an EMP.... helluva good question. Nuclear physics and atomic chemistry both baffle me a bit outside of how to make the things that go boom, so I don't even follow the math that suggests a non-nuclear explosion of that magnitude would result in a release of alpha and/or beta particles. I just know that there is a theory that suggests it. I'd doubt it though.