Fast pyrolysis of biomass is a next-generation biofuels production process that is capable of converting solid lignocellulosic materials (in their raw form) to a transportable liquid (bio-oil) which can be catalytically hydrogenated to fuels and chemicals. While biomass fast pyrolysis has enormous potential to produce renewable fuels, an understanding of the fundamental chemistry that converts biomass components, such as cellulose, to bio-oil is not available in the literature. In this work, we use thin-film pyrolysis to reveal the effect of temperature under transport-free reaction conditions and then evaluate the effect of sample dimension (i.e., characteristic length scale) by comparing product distributions of conventional powders and thin films. In the first part of the work, we show that the yield of total furan rings (i.e., all products containing a five-membered furan ring) does not change significantly with increased reaction temperature compared to other pyrolysis products, such as light oxygenates and anhydrosugars. However, we find that the functional groups bound to the furan ring (e.g., alcohols and aldehydes) are easily cleaved to produce smaller furans. In the second part of the work, we show that sample dimension is a key descriptor for product yields. For example, levoglucosan (the most abundant product of cellulose pyrolysis) yield differs significantly between conventional powder (millimeter-sized samples which are transport-limited) pyrolysis and thin-film (micrometer-scale thin-films which are isothermal) pyrolysis (49% for powder; 27% for thin-film at 500 C).