There is no right size plug. The human anus can stretch wide enough for a human hand to be pushed in, and in more extreme cases, both hands can be pushed into a human anus; as some of those specialist porn films will demonstrate. Having said that, just because it can be done, doesn't mean there is a good idea.
1.2 inches doesn't sound too bad, and stainless steel is a pretty safe metal, even for people with metal allergies, as stainless steel is inert, is easy to clean, and will not rust. I've never used stainless steel in any of my orifices, and I find that latex part plugs feel nice for me. All of these things are available on mile order, and, for those countries where these things are illegal, the mail-order companies usually ships in plain packaging. Whether or not one can afford them is probably the biggest problem.
You can wear your butt plugs for as long as it feels comfortable. Make sure it is a proper but plug, so that you cannot "swallow it" by accident. I think most of us have heard of horror stories where somebody has pushed something up his or her bottom, and then couldn't get it out again without a trip to the hospital's emergency room, with all the resulting embarrassment that caused.
I hope you find this information useful,