As Seeta and Karthik mentioned, the distance in all the directions play a crucial role. The exact computational size required to accurately capture the physics would vary depending on the problem. But as a general guideline, maintain the upstream and radial distance at 5 chord lengths and the downstream distance between 5-15 chord lengths. You can play with these numbers and see the corresponding effect on the flow field.
Attached below is a paper which uses 1.3L in the upstream and radial directions and 5L in the downstream:
https://www.sciencedirect.com/science/article/pii/S2468013316300729
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