Go at least a quarter inch smaller in diameter than the largest thing you've taken comfortably. Glass is hard, and if you can take 1.5" in diameter of silicone or some soft material, then 1.25" in diameter or less will likely be okay for glass or other hard material.
Of course, make sure that the dildo has a suitable base or a suitable length (at least 8", I think, is the recommended... someone can correct me on this) before putting it in your butt, so that it doesn't slide all the way in you.
While glass is seamless, it is unforgiving, unbending, and unyielding. There's no squish. At all. I suggest that starting with silicone is the way to go. Most silicones have only visible seams (if any seam at all) and won't be felt in use.