Please pardon my higgorance but what is 'fetching' ?- I've never heard of it before.
As you're talking about a swatch, I don't think it means something to do with a dog and newspaper but that's the only fetching that springs to mind.
To answer your question, for the last few weeks I've been trying out needle sizes and different weights of wool. Depending on your tension, you would definitely get away with using worsted (Aran) weight wool on 5mm and 5mm.
Right now, I'm knitting myself a cardigan using 5.5 mm needles in an Aran weight and I've just started a hat with two strands of Double Knitting (sports weight) on 4.5 mm.
Hope this helps.
All Best Wishes
For Double Knitting weight, 4.5mm is no problenm at all.