Each label size carries its own layout: which fields print, which line they sit on, how large they are, and how big the barcode is. You set it once and every label you print on that size afterwards follows it.
Change the layout
Where each field sits and how large it is gets set once per label size, in Settings > Labels, and applies to every label you print on that size afterwards.
A field you place on a line prints. Once your layout gives the vendor, cost, supplier SKU, bin location, printed date, lot number or the crossed-out compare-at price a line of its own - or the SKU, on the small stocks (Dymo 30332, Zebra 1.25 x 1 and 1.25 x 1.25, and your own sizes under 1.5 in wide), where the SKU starts unticked - its tickbox in the print dialog shows ticked and greyed out - placing it is already the instruction, so there is no second switch to remember. The tickboxes still work as before for everything you have not placed: tick one and it joins the small detail line. Title, variant, price, SKU and the barcode stay tickable either way on the larger stocks, so you can still print a run without them. To leave a placed field off, take it out of the layout.
Move fields between lines. Each line holds one or more fields - put the product name and variant together, give the price a line of its own, and so on. A field can only be on one line at a time.
Set a size per line - Auto, S, M, L or XL. These are ceilings: a long value still shrinks to fit rather than being cut off. This is why raising a size sometimes changes nothing - if the label is already full, there is no room to grow into.
Use the whole label - grows the text to fill whatever space the label has left over, instead of leaving it small in the middle. The barcode keeps the height you set, and nothing grows wider than the label. Turn it on when your text looks lost on the stock. For a bigger jump than that on a small label, the space has to come from somewhere: remove a line you do not need and drag the barcode height down, and the remaining lines get noticeably larger.
Resize the barcode - its width and height, and whether the digits print underneath. On a narrow label the bars can only be a whole number of printer dots wide, so the width slider moves in steps and can sit still across part of its range; retail barcodes (EAN and UPC) get their own, smaller quiet zone, so they take every dot the label has room for. The height slider's ceiling follows the label - about 60% of its height - so a 4 x 4 inch shelf label can carry bars 2.4 inches tall, while a one-inch roll stops at 0.6.
Add your own text with Add text line - your store name, "Made in Canada", a phone number. It prints on every label of that size and has no checkbox in the print dialog, because typing it is already the instruction to print it. Up to 40 characters; clear the box to remove the line.
Print your own Shopify metafields. The field list also holds your product and variant metafields, so you can put your own stored information on a label and position it like any other field. You pick from the metafield definitions your store already has, so there is nothing to type. A variant's own value is used when it has one, otherwise the product's value covers all its variants, and values are read at the moment you print. Up to four per layout. Text, numbers, dates, money, ratings, weights and dimensions can print; metafields holding a file or a reference to another record are not offered, because there is nothing readable to put on a label.
A live preview beside the editor shows the real label as your printer will render it - barcode width included, at the printer resolution you have set (see Print straight to a Dymo or Zebra label printer). The smallest stocks keep fixed layouts - there is nothing to move at that size - and images are not supported yet, so a logo or an address block still needs a taller stock than any size available today.