I found a pattern that I think I can do, no new stitches to stumble over. However, I want it to be bigger. I want it to be looser around the neck.
Would I just cast on more stitches? Does it matter how many? How do I know how many more I can add without making the cabled section look weird?
If I’m reading the pattern correctly you would be increasing the rib repeats as there are only the 2 cables. (The pattern isn’t written in rounds and therefore hard for me to follow; I’m about to be throttled on my data speed and so didn’t watch the video.) For a 2x2 rib, increase in increments of 4. How many would depend on your gauge and how much larger you want it. Say for example you have 4 sts/in and want it 2" bigger, add 8 sts.
I am knitting with:
5mm/8US yarn, 24inch/60cm (5mm/8US) circular needles.
I just want something that won’t be tight on the neck, but is not a plain knit/purl stitch. I like the look of the cable.
I can’t find a larger, looser pattern that doesn’t call for more complicated stitches so I was hoping I could just add stitches to what I see here.
If I were to add 8 stitches (or however many) how does that affect what I do? Do I have to do more rounds, or add more to the cabled sections? This is so confusing. 
Yes, you can just add sts as GG suggests, in groups of 4. Those sts will add in more of the k2p2 ribs but won’t affect the cable pattern. You can follow the directions for the rounds: “rib to the end of the round” without having to worry that you’ve added in extra sts.
You don’t need to do more rounds. In your case each round will have more sts than the roiginal pattern. Think of it as making a larger circle.
I looked again and it really does look like it’s just the 2 sets of cables, is that correct? If so, to make it larger in diameter just add stitches in multiples of 4, based on your gauge, to make it the diameter you want. If you want it longer (wider? deeper?) you could add more rounds of ribbing or add enough rounds to add more cable repeats. It all depends on your gauge and what you’re wanting. My math-impaired brain is more math impaired than usual this morning so I can’t help with exactly how many sts to add. Sorry.
Awesome, thanks! I am terrible at math, math makes me anxious, my brain goes blank. There might be a meeting of a knitting club in my city tonight, and if it is still on I am going to ask someone to help me understand how to read a pattern and do the math.
If I wanted to add to the cables like you said would I just follow the cable rounds with an extra cable round or two to make it larger?