OK, maybe not size per se, but gauge. And don't always trust your memory that you know what gauge you got the last time you knit with a certain yarn or needle. And don't believe yourself when you say, "Oh 72 stitches is perfect for a hat on size US 8 needles with this yarn. I've made them like that before." Because you know what? It's all a lie! Your brain will lie to you in order to not make gauge. It will convince you that the last time you knit with this particular yarn you got 4-5 stitches per inch. I'm quite sure of it. Yeah, and ... and ... and ... I've made lots of hats by casting on 72 stitches. Remember?
Except the sad truth is, now thinking back I've never used Moda Dea Wool 'n' Silk and all the 72 stitch hats I've made were on size US10 needles not US8 and I've never done anything like this before. Ever. Which all means that I just blindly cast on 72 stitches of a lovely blue silk and wool blend yarn and knit it like I knew what the heck I was doing. And in the end I had made a very lovely little lacy hat that would possibly fit a baby!
So repeat after me: Gauge matters. If you aren't 100% absolutely positively 100% sure that you know your gauge with that wool then don't just willy nilly cast on and set to town. You will spend many hours making something that doesn't fit anyone in your household (let alone the charity I was intending this hat to go to). It will make you cuss. It will make you spit. It will make you want to attack your finished work with a butcher knife. (Please refrain. I did and so can you, but you still want to.) Just suck it up, cast on some stitches and swatch for gauge. Your inner self will appreciate you for it.
Fortunately, I know a little boy who is going to look just dandy in a blue silk and wool cap and since he is a baby he wont' mind the lace.