The key as mentioned by subzzzero is frequency rather than size. Stretching both vaginal and anal is achieved best by long periods with moderate size plugs or dildos, preferably daily. This allows the tissues to relax and conform to the size without risk of tearing. Buy some graduated size butt plugs from Amazon and start out wearing the smallest one as much as possible, as you get used to it move up to the next size and so on. I know plenty of subs that wear a butt plug to work, now I know that isn't always possible but even if you aim for always when you are home you will quickly notice the effect.