This really depends on what you want from a knife.
Let's start with the steel. The carbon is nice. Very close to 1085 and hardened around 56Rc. It's tough, will dent when you hit a staple in the cardboard you're cutting but won't chip. It's made to be touched up regularly and easily. Responds to about any sharpening but really pops if you stick to convexing the blade.
The Inox is harder and run up in the 58-59Rc range. It holds an edge noticeably longer and really shines when sharpened to 2000 grit. If it's less tough than the Carbone, I've not encountered it yet but it wouldn't shock me either. The Inox is better with food as it imparts no taste to fruit and of course, can be put away wet. I'm a recovering carbon snob but would happily continue to Inox. It feels like harder Carbon (e.g. Schrade USA's 1095). Very comparable, in fact. I like it better than Buck's 420HC, fwiw, and really like Buck's 420HC as an everyday steel.
In terms of size, the #8 is the most popular size. Think of it as a folding paring knife. I wear XL gloves and find it to be a 3.5 finger hold, very similar to the Buck 112 in the hand size wise.
Buck 112 & Opinel #8 by
Pinnah, on Flickr
For my XL hands, this a bit too small but doable. If you have L gloves, it may be the size for you. Pocket carry suitability will depend on what you wear. I wear baggy carpenter jeans and carry them lock ring down in my RFP and don't notice the #8 or #9 at all.
For EDC use, I prefer the larger #9. The blade is wider spine to edge and the handle gives me a full 4 finger hold. Here is a set of slightly modified Opinels.
Opinel by
Pinnah, on Flickr
For me, that little bit of extra room gives me a much better grip with the #9.
The #10 is just a bit too much for pocket carry. I notice it. I like it, in hand, and when doing yard work, I'll reach for it. I prefer it in the kitchen and will carry it as a light but tough back up knife in my pack. But it's also a bit too big socially in my suburban life. The blade is like a Buck 110. People do notice it.
My son has a #6. For me, it's a 3 finger knife. If you want a small pocket knife it's worth looking at. The #7 is like a mid sized pocket knife. Sort of a "tweener" size.