R. JUDAH B. BABA FURTHER RULED: IT IS PERMITTED TO MOVE OBJECTS (on the Sabbath) IN A GARDEN OR A KARPAF (storage area) WHOSE [AREA DOES NOT EXCEED] SEVENTY CUBITS AND A FRACTION (2/3) BY SEVENTY CUBITS AND A FRACTION (2/3).
There are other requirements, including the size of a wall and the presence of something to indicate that it is a “dwelling.” For now, we’ll just concentrate on size.
R. Akiva agrees with this size (70 2/3 x 70 2/3 cubit area) although an earlier Mishnah (on 18a) has the area as “two bet se’ah”.
There is a marginal difference between the two. But where does this numerical value come from? Torah:
The length of the (Tabernacle) court shall be a hundred cubits, and the breadth fifty everywhere (Ex. 27:18)
Creating an area (100 x 50 = 5000 sq. cubits) similar to the area described here (70 2/3 x 70 2/3 = 4992 sq. cubits).
But this brings the question: what is “fifty everywhere” (chamishim b’chamishim). Divide the 100 cubit length in half, creating two fifty cubit squares. Then:
Abaye replied: Put up the Tabernacle (which is 10 wide x 30 cubits long) at the edge of fifty cubits so that there might be [a space of] fifty cubits in front of it and one of twenty cubits on every side.
The Tabernacle would thus sit inserted into the courtyard with its entrance at an imaginary 50 cubit line bisecting the 100 cubit length. It would be centered inside the 50 cubit width with 20 cubits on each side. The back of the Tabernacle would then be 20 cubits from the back of the Courtyard wall.
It helps if you draw it! (at least it helped me.)