Would it make much of a difference in the finished article if I used a 10.5 circular needle instead of the 11 circular needle called for in the pattern?
Yes, it would make a big difference. Size 10.5 has a 6.5 mm diameter while size 11 has 8 mm. If you were to knit 10 sts on size 10.5 needle, you would produce a 15 mm (a little more than 0.5 inch) smaller article than what the pattern suggests. You probably could get by with making a scarf if you don't worry too much about the width. I would suggest investing on interchangable citcular needles because it has so many options of needle sizes and cable lengths to choose from. It worths it for the long run. Make sure to do enough research before buying one. I hope this helps. Sorry for the delay. Just moving to a new place and I don't have a good internet connection yet. Have to work from my phone. Thanks for the question. Ratcha