mesquite- Your entire rectum needs to be stretched for that. You rectum is filled with a series of muscular rings much like your sphincter and each one of them needs to be teased open to accept a large object through them.
tuscon- don't use a blow up plug to force your sphincter open. You need to learn to relax that muscle and open yourself up to accept larger sizes. Your sphincter is a muscle and if you learn to control it well you can get fucked with bigger dildos/toys and you also will be forever safe from hemmorhoids and incontinence because that muscle will be strong and your will know how to control it very well without having to think about it. Forcing your sphincter open, be it with a blow-up toy or sheer force, is a bad idea. Instead get a tapered toy where you can start out at 1.25" and then focus on relaxing and gently taking more and more of it over the course of a few hours until you get to 1.5" and then keep doing that until you can enjoy the size you desire. It might take a few sessions with yourself or a partner to get there, but the hard work will be worth it in the long run because once you learn to relax and expand that muscle, that's a skill that you will have and can keep enjoying! Damaging your sphincter by forcing parter objects than it is ready to handle will not help you out in the long run... Good luck with your exploration!!