what size needles

Im knitting a cardigan using two strands of Debbie bliss Mia wool, single strand says use 4mm for double what size needles shall I use, I was thinking a 6-6.5 or 7 mm
I tried 5mm and they were too tight im not using a pattern and am doing this freehand

Welcome to KH!
The rule of thumb for this is to double the needle size in mm that is recommended for the yarn. So that would be 8mm but it’s just a rough guide. You’ll really have to swatch with different sizes to see which you like.

Clearly my maths isn’t up to scratch and that didn’t even cross my mind, cheers Xx

Oh, I think it’s something that seems to work over a certain range of needle sizes. I’m not sure that there’s any math involved just folk wisdom (someone else’s).