There was a good discussion of this on Reddit recently. Sorry to link to Reddit, but it's a good, topical post worth perusal.

https://www.reddit.com/r/Agriculture/comments/1dv7fw9/how_much_good_land_is_used_to_grow_food_for/

ETA:

We recommend four widely applicable high-impact (i.e. low emissions) actions with the potential to contribute to systemic change and substantially reduce annual personal emissions: having one fewer child (an average for developed countries of 58.6 tonnes CO2-equivalent (tCO2e) emission reductions per year), living car-free (2.4 tCO2e saved per year), avoiding airplane travel (1.6 tCO2e saved per roundtrip transatlantic flight) and eating a plant-based diet (0.8 tCO2e saved per year). These actions have much greater potential to reduce emissions than commonly promoted strategies like comprehensive recycling (four times less effective than a plant-based diet) or changing household lightbulbs (eight times less).

^https://iopscience.iop.org/article/10.1088/1748-9326/aa7541/pdf^