Online orders and sizing

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Okay so, I'm going to be able to order online soon and I have a question about sizing. I took all my measurements and this is what I need to know; when it comes to ordering online, how much size difference can you get away with? I imagine going smaller even by an inch than your measurements would make it too tight, how about how far when going bigger? This would be for non shirred items. Does it not make a difference and just depend on the brand? Any info is helpful! Thanks!