My first peplum top was a size too small because I didn't understand what the flared bit was meant to do, so it sat like a stiff little skirt around my ribs instead of skimming over my middle. The thing it taught me is that a peplum needs to actually clear your body to work, it can't be fighting its way out from underneath you.
Getting the fit right starts with the seam where the peplum joins the top, that needs to sit at the narrowest part of your middle, not above it and not at your hip. Too high and it makes you look boxier than you are, too low and the flare has nowhere to go but cling. Try lifting your arms and sitting down before you decide on a size, because a peplum that looks neat standing still in a changing room can ride up at the back the second you bend forward over a desk or a car boot. I size up more often than not with these, since the bodice part tends to be cut close and the flare is where the fabric has room to move anyway, so a bit of extra space up top doesn't throw off the shape. Watch the shoulders too, a lot of peplum tops have set in sleeves with no give, and if you've got any width across the back at all you'll feel it pulling the moment you reach for something on a high shelf.
It's not a top for every day, and I think the high street oversells it as one. On anything with a very structured peplum, the stiffness that makes the flare stand out nicely also means you can't layer much under a coat without looking bulky, and it's not forgiving if you're sitting at a desk for eight hours, that seam at the waist digs in after a while in a way a plain tee never would. It also wants a bottom half that isn't doing much of its own, a fitted skirt or straight trousers, because peplum on top of anything else with volume just reads as a lot of fabric in a lot of places.
The ones with a bit of stretch in the weave wear better over time than the fully structured cotton versions, which can go papery and lose their stand after a few washes, the flare drooping instead of sitting out. I turn mine inside out for a cool wash and hang them rather than fold, because folding puts a crease right across the peplum seam that's a pain to steam out. Good ones soften at the bodice with wear but keep their shape at the hem for a surprisingly long time, which is more than I expected from something this cheap when I first bought one.
This suits anyone who wants a bit of shape at the waist without a fitted dress, and wants it to still look like clothes by the end of the day.