It is important to choose the right stitch size for the yarn and stitch type that you are using in your knitting. Choosing a stitch size that is too big will result in an open, floppy fabric, while a stitch size that is too small causes a tight, rigid fabric and sometimes even breaking yarns.
Every yarn is different, and as such you should always make some swatches to find out what is the best stitch size for your yarn. After gaining some experience in knitting, you will be able to estimate stitch sizes more accurately for different yarns.
In the image below you can see a swatch with different stitch sizes. At the top, you can see an area with stitches that are quite loose. At the bottom, you can see stitches that are so tight it is impossible to see one defined stitch.
Unless you are working with a yarn that is quite heavy for our machine (Nm6), a safe stitch size to start with is 7. Make sure to use a slow speed when testing a new yarn.
It is possible to assign different stitch sizes to the front and back bed. See video below.