The effect of the sample width on the flame spread rate over a thin PMMA film in microgravity has been investigated by experiments and scale analysis. The oxygen level of the ambient gas is kept fixed at 30% while different diluents-N2, Ar, He, and CO2-are used. The sample width is also used as a parameter, varying through 5mm, 10mm, and 20mm. When helium is used as the diluent, the conductive heat loss to the sides becomes large, and therefore, the flame is suppressed with a significant drop in the flame spread rate and flame size, especially at low oxygen level and for narrow sample widths. For other diluents, the radiative loss is the dominant factor for the suppression, and the sample width has small impact. These experimental findings can be explained with the scale analysis.