I remember playing with magnifying glasses at school and we found the biggest diameter and the shortest focal length best to make fire. I've carried an old, chipped lens in my possibles bag for years and I know it works because I used to light my pipe with it when I lived in the tropics. Although whenever I've needed fire in a hurry it's usually wet, cold and overcast. I have a credit card sized fresnel lens in my wallet mainly in case I break my glasses, but even in ideal conditions I've never been able to make fire with it.
The older I get, the better I was.