Polyurethane anti roll bushes

[Ooga]

Hi.
I need to replace the anti roll bar bushes on 2010 ibiza.
The bars are 18mm.
I've seen that strongflex polyurethane bushes are not expensive and can be better, but I have 2 questions:
Do I need 16mm bushes or 18mm bushes (18mm bar has 16mm bushes??)
How do I Insert them? the original ones are open, and it looks like the polyurethane ones are closed and the bar has metal stops..
https://www.strongflex.eu/en/skoda-fabia-i-00-07/945-front-anti-roll-bar-bush-5902553513592.html

Thanks.