Reducing agricultural carbon emissions is important to enable carbon emission peaking by 2030 in China.  However, China’s transformation towards large-scale farming brings uncertainties to carbon emission reduction.  This study quantifies the carbon emissions from cropping based on life cycle assessment and estimates the effects of farm size on carbon emissions using a fixed effects model.  Furthermore, the variations of the carbon emissions from cropping driven by the changes in farm size in future years are projected through scenario analysis.  Results demonstrate an inverted U-shaped change in total carbon emission from cropping as farm size increases, which is dominated by the changes in the carbon emission from fertilizer.  Projections illustrate that large-scale farming transformation will postpone the peak year of total carbon emission from cropping until 2048 if the change in farm size follows a historical trend, although it is conducive to reducing total carbon emission in the long run.  The findings indicate that environmental regulations to reduce fertilizer usages should be strengthened for carbon emission abatement in the early stage of large-scale farming transformation, which are also informative to other developing countries with small farm size.