So what do you do if "dimensions of a roll of toilet paper" is a phrase that shows up repeatedly in searches leading to your site?
I've designed several covers for toilet paper rolls, and the patterns have one thing in common: they don't give specific stitch counts for the covers. Too much variation in rolls, both between brands and within brands - and over time, too. I can tell you what I did, but stitch counts and gauge that worked for my roll of Seventh Generation single-ply may not work for your double roll of two-ply Charmin. "Work until it's large enough" is annoyingly vague, perhaps, but more likely to result in something that fits the roll in question.
But... What if there is no roll to measure against? What if you're making this as a gift, and you don't know what toilet paper the recipient uses?
Horrors. What a calamity.
As I see it, you have two main choices:
Well, well, well... What have we here? A chart. With a few measurements. Just a few, mind. I have a small apartment, and I'm not about to buy samples of every make and model of toilet paper just to get measurements. There are limits to my closet space.
I'll add more measurements as I get them. And if you have measurements for a specific roll and would like me to add them to the list, please let me know.