Short answer:
The same size pen that you would keep any two sheep in. A 5ftx5ft space for each animal so 50sqft for 2 animal. Normally they would have 20-50 ewe and 1 ram in a mating pen, during the night and during the day the males and females would be sent to different pastures.
Long answer:
How many full size sheep do you plan on keeping on this pasture/pen? Is closer to 50 or just the 2 of them?
How many days will they be on this pasture? 3weeks or 5weeks or 7weeks aka 1 or 2 or 3cycles?
When was this area last harvested/grazed? 4weeks ago ensuring it is at max nutritional value?
Do you plan on doing rotational grazing during this 3wk - 7wk process? If so will you be doing daily moves?
What is the summer/fall carrying capacity of your land/pasture/pen? 1sheep is 0.2 Animal Unit, ram = 0.25
I am going to make some assumption (such a famous phrase).
You will be keeping 2 sheep (0.45AU), for 7 weeks, in a wonderful pasture with a carrying capacity of 3acres per Animal Unit.
So you will need 1.35 acres per month (0.45 x 3acre), but if you don't do rotational grazing you will need 2.3acres for 7weeks.