Thank you all for your comments. I ended up being busy this evening so I havenít got any further with the project, but when I got in from work, after spending most of the day researching instead of working, I did make a discovery: I thought it was really strange to have a 1:1 stitch/ratio too, so I decided to re-count. There is a chance *looks sheepish* that I may have misread the tape measure I had handy last night and the swatch I have is actually 13x16, which, although not correct, is still much closer to what I should be aiming for. I did check multiple times last night, but must have been reading off the wrong mark. I have also studied the wrong side of the work a bit and have noticed that some of my purl rows are not quite as even as my knit rows. The jumper will be knit in the round, so it will all be knit rows and if I intentionally work a little tighter than usual that might give me the 0.5 rows per inch. As tomorrow is a holiday here in Germany, I have decided to do a 5.5-needle swatch tomorrow anyway and see if that gets me where I need to be, but otherwise I might just stick to the 6 mm and learn to read the tape measure properly.
Just for info: I am working on ůrůi, pg 179 in Knitting with Icelandic Wool. I have a couple of the patterns in the book lined up, and this is my least favourite, which is why I chose it as my starter project. If necessary, Iím sure I could tweak the yoke pattern a bit, but Iím also tall, so it might even work out fine to have a little extra length
P.S. The tip about switching needles mid-swatch is a great idea for next time. This time I have already cast off, so itís just as simple to restart, but Iíll bear it in mind. Have a good day, everyone!