Math seems about right to me. Photosynthesis is only 3% efficient, vs. about 30% for current-generation solar panels, so that's already 320 acres. Current agricultural crops put about 40-50% of their incoming solar energy towards the edible parts, so that's 640 (you can thank the green revolution for that - wild plants put only about 5% of their incoming energy towards fruit/seed production, but we've selectively bred them for a 10x improvement in productivity). You lose a factor of about 10 going up a trophic level to eat meat, so if half your diet is meat, that'd be 3200 + 320 = 3520 acres to support your diet. Figure on transportation losses and food waste for the additional factor of 2.

All numbers are from a sustainable agriculture course I took in college, with a few spot checks by Googling.

This, BTW, should drive home just how environmentally-damaging carnivorism is and how switching to a vegetarian diet is actually significantly more impactful than almost any household energy conservation you do. However, as a long-time meat eater, I don't care, and just accept that I'm a terrible person.