I think aran weights are all close in thickness. You can absolutely experiment with needle size--if you for some reason don't like the look or feel of the fabric on different size needles, you can make a different size.
If a pattern calls for aran weight, I'd feel comfortable ordering aran weight yarn and taking it from there.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry