The size of the needle affects how big your stitches are, and therefore the size your finished product, will be. The gauge, or how many stitches fit into an inch of knitting, relies heavily on the size of the needles. In fact, if your gauge doesn’t match what the pattern calls for, the way to fix it is to change the size of your needle. Most knitting patterns are pretty clear when it comes to telling you what size knitting needles you will need and that makes sense, because the designer wants you to be able to recreate the project successfully. It would be a lot simpler, if knitting needles were a standard size, but in fact there are three (British, American and Metric). The American system starts with low numbers for needles with smaller diameters working up to larger numbers for larger diameters, the British system starts with high numbers for low diameters and low numbers for high diameters, and the metric measurements indicate the diameter of the needle in millimeters. The only place the American and British numbering systems agree is at 4.5 mm; both countries call that a size 7. One way of seeing that size matters, is to try knitting a swatch on a size 10 American set of needles and a size 10 British set. You’ll get two very different results! As seen in the picture below both were knitted on 20st over 20 rows, the top is the British size 10 and the bottom American size 10
So in knitting Yes! size does matter.