What fit-all-size means here
The collection is adjustable, and that is not a slogan smoothing over a narrow size range. It is how this kind of garment is constructed: the bodice closes adjustably rather than to a fixed measurement, and the part of the dress doing the work in the photograph is not fitted to anybody at all. Plus sizes are genuinely covered and regularly shot.
The practical consequence is that the thing people worry about most before one of these sessions is, in the ordinary case, not a problem. You are not choosing between a dress that fits and a colour you want.
The bodice, and why adjustable is not the same as universal
An adjustable closure covers a wide range and is not infinite, and pretending otherwise would be the kind of promise that gets discovered on a roof at six in the morning. This is why fit is still confirmed with you before the date rather than assumed.
That confirmation is a short conversation, not a fitting. It exists so that if there is anything to accommodate, it is accommodated in advance, which usually means the assistant knows how the back is being closed before the dress comes out of the bag.
Height, and the only measurement that really changes the picture
Length is the one dimension where your body and the dress interact in a way worth planning for. The train is long by design; what varies is how much of it is behind you and how much is pooled around you before the throw.
Neither case is a problem, and both change what the photographer does. Shorter and the frames tend to work better low and against sky, where the fabric reads as a shape rather than a puddle. Taller and the dress sits closer to the proportion it was cut for, which means more of the wide landscape frames work straight out of the camera.
Say your height when you enquire. It is more useful to us than a dress size.
Maternity
A maternity shoot is one of the clearest cases for this cut, and it is why the maternity page answers the sizing question rather than deferring it: an adjustable bodice over an unfitted train accommodates a bump without alteration and without a separate garment.
What still gets confirmed is the same as for anyone else: fit and colour availability for your date. What does not need discussing is whether a dress exists.
What to wear underneath, briefly
Two things matter and the rest is comfort. Wear something seamless and in a tone close to the dress rather than contrasting with it, because an adjustable back and a bright strap are not friends. And wear footwear you can stand on uneven ground in for forty minutes. Nobody photographs your shoes in a five-metre train, and a rooftop or a dune is not a floor.
The sentence to take away
The dresses are adjustable and cover a wide range of sizes. Which one suits you, and in what colour, is confirmed with you before the date rather than assumed, because availability is per date and because a surprise at six in the morning is worth one message the week before.