By eyeballing my memory (how's that for a catchy phrase) I'd say you can escalate with wood 2, 3 or maybe 4 times the size, but not more. If all you have are biggish pieces either split them completely or use a knife to splinter one edge ragged, and set them "standing" to let the flames lick along the edge. Smooth surfaces catch fire less easily, because there's less air going around, I presume.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett