You are asking help and advice(s) but you don't help members to answer you. Members have to search on the web what you are talking about, no one of us knows all the toys available on the market. So please help people to answer you and give details and links, following this way you will have more chance to read more answers about exactly what you asked help or advise.
Is this one the toy you talking about ? http://www.docjohnson.com/buttplugs-smooth-classiclarge.html
Why do you immediately begin with a large plug ? use first a medium one and enjoy the feeling, learn to enjoy and after multiple exercises when the medium don't offer you the same strong pleasures it will be time to change for a large one, so you will enjoy two time more pleasurisable experiences and moments. Also a medium plug will be possible to be shoved without pain or difficulties.
Remember your body have to exercise and learn, this will not be done by snapping your fingers, go slow and progressively, go soft, take the time to enjoy.
The pain is a alarm signal of your body, a way to let you understand you are not ready for doing what you try, exercise progressively first.
Without preparation nobody can run the iron man race.
Buy a medium plug and play with it, http://www.docjohnson.com/buttplugssmoothclassic-medium.html this one will slip easy in and out, nice to stretch your ass sensually.