In this case, labels are printed on individual sheets in the required format (A3, A4, A5 etc.). The larger the number after the 'A', the smaller the sheet; in fact, paper size is halved with each numerical increment. Sheet labels are usually designed for use with laser and inkjet printers.
When it comes to actual label size, the range is limited (at least as far as standard labels are concerned) to certain standard dimensions determined by the intended use. Of course, the aim is to include as many labels as possible on each sheet. In the case of address labels, it is only really possible to divide an A4 sheet (210 mm in width) into two or three columns, otherwise the labels will be too narrow. The same considerations apply to label height.