In my experience it's better to size up.
I own three sweater from drake's, two original fair isle and a donegal aran from years ago
All of them are tagged M/42, they are perfectly comfortable, slim and not baggy.
Talking of common brands, I wear Size M in Polo RL Knitwear, and 38/40 in Sportcoats, Polo by Cornaliani & other italian brands.
Note that the Aran you are interested in, is probably made by Inis...