Using a 101/2 needle in place of 11 needle.
The pattern calls for an 11 circular knitting needle .
I have one 101/2-how much difference will it make in the finished piece?
It will depend on how tightly or loosely you normally knit. I am a tight knitter so if I was to use a smaller needle the finished article would be considerably smaller.
The best way to work out what the difference would be is to knit a small tension square
and then you can compare the number of stitches and rows per 10 centimetres (or 4 inches) to the tension specified in the pattern.