what Nutella was meaning is that the sizes on the packets are as meaningful to your ready to wear/shop size as the colour blue.
a lot of people new to dressmaking look at the pattern packet and pick the same size that they would try on in a shop. I'm a size 14 in ready to wear clothes, in the 'big 4' pattern company patterns (vogue, simplicity, burda, new look) I'm anything from an 18 to a 22 going by the packet measurements, but more like a 16 to 18 on the finished garment measurements. In Tilly and the buttons I'm a size 5 or 6. In closet core I'm a 12 - 14. In I AM patterns I'm a 44.
So basically the sizes on the packet bear no resemblance to UK shop sizes and differ from pattern to pattern even within the same brand of patterns. In the 'big 4' pattern companies there is often also a lot of wearing ease built into the pattern (basically they are quite a loose fit) so if you like something a bit more tightly fitting you have to check the finished measurements on the actual paper pattern pieces to see which ones might be closest to your actual measurements.
some of the indie pattern companies have moved away from tradition size numbers and perhaps number the pattern sizes from 1 to 8 rather than 8-20.
lots of people also don't pull the tape measure tight enough either which can result in getting the wrong pattern size.
my advice would be to start with an indie company like Tilly and the buttons or Ellie and Mac which have very straightforward sizing and very clear instructions.