Knitting swatches

I’m knitting a pattern in Aran yarn calling for 5mm needles. I followed the instructions to knit the swatch using 5.5 mm needles and it came out a centimetre too long. Should I knit a smaller size? Should I knit on smaller needles eg 4.5? Would appreciate any help. Thanks.

Hi and welcome!
When you say that the swatch came out a cm too long, do you mean that it was a cm to wide or too long?
Be sure to measure over the middle 10cm of the swatch and don’t include the edge sts which are smaller and will throw off the gauge.
If the gauge swatch is too wide, the stitch gauge can be adjusted by trying the 5mm or 4.5mm needles. If the swatch is too long (the row gauge is off), that’s usually not a problem as most patterns give length in measurements rather than in numbers of rows. As long as the stitch gauge is good, you should be fine.