GAUGE or TENSION!
The gauge is the key to a proper size of your garment but also to its durability.
Always knit a sample swatch!!
A Danish designer Marianne Isager once said: I cannot believe why some knitters are so hard-working that they complete a garment before they realize that the gauge were not OK.
You might think that you do not need to knit sample swatches because your gauge always turns out well. But even if your gauge is quite "normal" it might differ from the gauge specified by my instructions and you may end up with a garment which is either too big or too small - and you might need more yarn.
... it is not enough to measure the number of sts the 5 or 10 cm (2" or 4") - you also need to count the number of rows. If you stretch your sample a bit in the width you will get more rows - and if you stretch it in the highth you get less rows. So it is a combination of stitches and rows.
When the gauge specified is 26 sts and 32 rows to 10 cm / 4”, in garter st on needles 3 mm/US 2-3, I suggest you cast on at least 32 sts and work 42 rows. Bind off.
(The sample photographed is a random sample from my stock of samples.)
Place the swatch on a table, flatten it without stretching. If your swatch has more than 26 sts and 32 rows to 10 cm/4”, use smaller size needles, if it has less sts use larger needles. It is important to reknit the gauge swatch and remeasure. The needle size indicated is only a recommendation.
The pattern here is almost squared, but if you pattern should be a stocking stitch pattern your gauge will probabaly be more like 3/4 - for example: 26 sts x 34 rows the 10x10 cm (4"x4").
What really matters is that your gauge is correct. Use whatever size needles you need to get it right. Not two persons knit the same way. You may even have different gauges depending on what mood you are in. Measuring gauges when knitting the Domino knitting is not always the same. See the instructions.