Sizes are options, not separate listings
One item, one product, sizes as options underneath. Splitting the same leggings into five listings makes the catalogue unreadable and the stock count dishonest, because the seller updates one and forgets the rest. With options, a size that runs out goes quiet on its own.
Put the fit in words, not just the size chart. Whether something runs small, whether the waist is high, whether the fabric gives. The people who ask you this are the people who have not ordered yet.
Photograph movement
Sportswear is bought to be worn in motion. A flat photo shows the colour and nothing else. One shot mid-stride or mid-lift tells the buyer how the fabric behaves, and it does more than another close-up of the logo.
Restocks are your best message
Keep a note of who asked about a size you did not have. When it lands, message them before you announce it anywhere. That message converts better than any post, because you are answering a question they already asked.
A sold-out size that stays visible is part of this. It tells you what to reorder, and it tells the buyer you are not hiding.
Sell around the training week
Announce drops when your audience is planning, not when they are training. Evening posts before a weekend do better than a morning during it. The shop button in the bot menu is the way in, so the announcement can be one line without a link buried in it.