Linen dresses used to mean one shape, loose through the body, loose through the arm, and a colour range that stopped at oatmeal and white. That's changed a lot in the last few years. There's proper tailoring in linen now, shirt dresses with a waist, slip styles, midi lengths with enough weight to them that they don't just hang like a sack. I think that's a genuine improvement. The old version had its charm but it only really suited one body shape and one occasion, sitting in the shade with a cold drink.
Fit is where linen catches people out, because the fabric itself doesn't help you. It has very little stretch and almost no give, so if a dress is cut close anywhere, across the bust, under the arm, it will show every bit of that tightness rather than easing into you the way a jersey would. Go a size up if you're between sizes, especially in anything with a fitted waist or a button front, because linen creases exactly where it's tight and that crease stays there all day. Watch the shoulder seam too. If it sits even slightly low, the whole dress drags forward when you walk, and that's not something steaming fixes. A dress cut generously through the body with some structure at the waist, rather than something skimming or clinging, tends to forgive a lot more.
If you've never owned one, start with something in a midweight linen, not the thin, almost see through stuff, in a midi length with a bit of a waist defined, either a tie or a seam, somewhere between loose and fitted. That middle ground works for more days and more shapes than either extreme does.
In the shop, check the lining if there is one, because a fully lined linen dress behaves completely differently from an unlined one, less static, less clinging, but warmer. Check how the hem sits when you move, since linen with no weight will flip about in wind rather than swing. Have a feel of the weave too, a slightly slubby, textured linen softens with washing and wear, where a very smooth, flat one tends to stay stiffer and show every crease forever.
For everyday wear, a plain linen dress sits well under a denim jacket or a cardigan on cooler days, and takes flat sandals or trainers just as easily as anything smarter, so it earns a place beyond the height of summer. Wash it on a cool cycle and hang it damp, and most of the ironing takes care of itself.