Reference for Eruvin 47:5

נטע רובו הרי הוא כחצר ומותר: אמר רב יהודה אמר אבימי והוא שעשויין אצטבלאות ורב נחמן אמר אע"פ שאין עשויין אצטבלאות

[it follows that] if its lesser pa [only was sown the movement of objects within it] is permitted. Said R'Huna son of R'Joshua: This applies only [where the sown area was no more than] two beth se'ah but if it was more than two beth se'ah<span class="x" onmousemove="('comment',' Since the enclosure was not put up for dwelling purposes.');">¹²</span> [the movement of objects within it] is forbidden.

Jastrow