As has been said already, it's really the prep that makes the difference. Fine, fluffy, airy. Maximize your surface area. But to answer your questions, based on my own experience:
1) A nice size to start with is around that of the Light My Fire Army model. 3/8" diameter and about 2 1/2" in length. It provides a good length when you're first learning, and 3/8" will last a long time. Smaller rods are better for being compact and lighter, but for someone just starting out, at least that size and length is advantageous. When you go shorter, you tend to need better technique since you won't generate as many or as intense sparks per strike. And the sparks you do generate, probably aren't as hot or long-lasting as the sparks you get when you really dig into a big firesteel.
2) There is no "better" brand, but I've had good experience with firesteels from Light My Fire (LMF) and GoingGear (GG). Composition of the ferrocerium rods vary by manufacturer, and different firesteels will require you to modify your technique a little. For example, you need to strike an LMF firesteel a little harder to get lasting burning globs, but you can get smaller sparks by fast and light scraping. In contrast, GG firesteels require quick hard strikes (they seem softer, for lack of a better description), but you get some beastly molten globs. I also tend to the prefer the handle shape on the LMF firesteels, but the handles on the GG ones tend to be larger so they may feel better if you've got larger hands.
3) I prefer to use either the dedicated striker that comes with the steel (make sure it's the newer flat-edged strikers, if you get a LMF steel, not the older serrated/ridged strikers), or one of the awesome little strikers from GoingGear. Smaller striker = better control when striking. Neither weigh much and are easy to carry with your steel or on a keychain or something. Another popular option is a piece of hacksaw blade. Cpurcell is on the money with the spine info. To echo him, you don't have to strip the entire blade, just clean off a section of the spine with sandpaper, and make sure the edge is squared and crisp. As long as the material is sufficiently hard and has a crisp enough edge angle (acute or right, doesn't matter), it will strike. Stainless or carbon doesn't matter.
And yes, I wouldn't use the edge of my knife unless I had no other choice. I'd rather preserve my sharp edge. If you do use the edge, it's not the end of the world though. At most, you may dull a little bit of the edge closest to the handle. You'll still have plenty of sharp carving space closer to your hand and the rest of the blade farther from your hand. Anyway, I prefer to use a dedicated striker because it allows you to plant the tip of the steel into a firm spot under your bundle and really bear down on the steel with control and without the blade getting in the way. In my experience, harder strikes that throw a dense shower of sparks and/or burning globs of the ferrocerium work best when igniting tinder. Holding the knife/striker steady and pulling the firesteel against it will give you a less intense shower and usually no globs, at least not in a controlled, directed way. Just think of how much pressure you can exert on the firesteel that is braced against a solid surface vs. the pressure you can exert on a firesteel you're holding suspended in the air (AND think of which one gives you better control of where the sparks land). But ultimately, different strokes for different folks; give either method (or any of the others) a try and find out what works best for you :thumbup:
This is useful primer on firesteels and their usage, by BF member Evolute. It's a good little read and may provide you with additional info.
Have fun!
