Just a quick bit of science before we all get carried away with magnifications.
To ignite something flammable you just need to make it hot enough in air. To increase the temperature of whatever it is you're trying to set fire to, all you have to do is arrange the condition that more heat is entering it than is leaving it. Aside from some really outlandish chemical reactions which we aren't ever going to see in the wilds, if more heat is entering something than is leaving it then the temperature will rise. While this condition holds the temperature will continue to rise. All you have to do then is wait.
To recap, you need two things: as much heat as possible entering, as little heat as possible leaving.
It doesn't really matter how much material you're heating, but since more material needs more heat to heat it, and since you're working with a finite amount of heat -- whatever you can coax from your magnifying glass or mirror -- and since things conspire against you all the time, you probably want it to be a modest amount of material. Of course there needs to be enough that when it does burst into flame it doesn't burn itself out before you get a chance to light a fire with it. A bit of experience in differing conditions will give you a feel for it, I like to try to light something about the size of a five to ten millimetre length of a pencil.
Other things (e.g. how sunny it is) being equal, the amount of heat you get is proportional to the area of the magnifying glass, so the bigger the the glass better. If it's TWICE as big across you get FOUR times the heat. If it's THREE times as big you get NINE times the heat, and so on.
The "magnification" is irrelevant. It doesn't matter how many times it says the magnifier magnifies, pretty much all that changes with the magnification is the focus distance. You might even be better off with a slightly longer focal length (lower magnification) because you won't have to hold it so close to the fuel, so your glass might not get so smoky, and you'll also be able to see what's happening that bit easier.
It DOES matter however about the quality of the lens or mirror. For a given size of glass, you heat up a small spot hotter than you heat a large spot. Some of the cheap Chinese glasses (well, often plastics) are of very poor quality and you can't easily get a very small spot with them, although if the sun is strong enough they will still be good enough. Jeweller's loupes and linen testers are usually pretty good optically but it pays to check. Look through it at something like graph paper, if the lines look all wriggly go elsewhere for your optics. But a loupe is usually very small, and while it might work in strong sun it might not be so effective if it's hazy.
So much for putting heat in.
You also have to let as little heat out as you can. I try to arrange that whatever I'm lighting sits on an insulating surface and I'm aiming the spot on something which you can imagine as the inside corner of an insulated box which has had three sides removed. The insulation can be fuel material like wood or paper. That way some of the heat which inevitably escapes while you're working on it will heat the sides of the box near to the spot, and some of that heat will be reflected back into the corner. You can make a hole or a depression in the fuel, or use a short section of wooden tube with a bung at one end, anything that comes to hand. The shape doesn't really matter as long as you keep as much of your hard-earned heat as possible from escaping. Whatever gives you a small, sheltered spot to heat will do. Little rectangular blocks work fine for me. Even if the fuel is wet, you can dry it out if you're patient and you have enough heat input.
If you do have to blow, blow gently.
