Whats the difference between the sizes, 1.9-1, 1.8-1, etc..... Obviously i know what the literal difference is but whats the difference in application of them?
Is there "safer" ones to used between the different sizes?

 

There are really only two options I'd bother with on rockers at this point:
-if you're going to run an aftermarket cam, go with stock rockers
-if you're just doing bolt ons and don't plan to do a cam, pick up some modded stock 1.9s

In general, the more lift the more power can be made since the valve can let more air pass during each opening. However you can only run so much lift before the spring binds or the retainer hits the valve guide/seal.

Rockers add some power and are easy to install. However a cam will make more power than rockers alone since it can adjust ramp rate and duration in addition to lift. It's also far harder to install.

I'm just not a fan of spending big bucks on pretty aftermarket aluminum rockers anymore, especially on these engines. The stock ones are super reliable and the 1.9 modded stock ones can be had for under $200 used. Unless you go with some expensive shaft style rockers for a crazy all out build, stock is where it's at.