UPDATE: The below is only applicable to OOTP6.5. We are in the process of coming up with rules for stadium modifications, as well as when they can be modified and how. I'm leaving the old for reference.

The numbers listed below are in reference to the Ballpark Factors found on the Edit Ballpark page (they guide how hitter/pitcher friendly the park is, how easy it is to hit homeruns, etc). Splits refer to lefthanded and righthanded batters specficically. For instance, you can have a 115 average for lefties, and a 110 average for righties, and be 112 overall. That would be within the guidelines.

Average:
No lower than 85 overall, no higher than 112 overall. Splits may be no lower than 80, no higher than 115)

Doubles:
No higher than 120 overall, no lower than 80 overall.

Triples:
No higher than 125 overall, no lower than 75 overall.

Home runs:
No higher than 117 overall, no lower than 75 overall. Splits may be no lower than 70 or higher than 120.

Here are the other aspects of a stadium that you can edit:

• Name
• Wall Dimension (how many feet from home)
• Wall Height
• Avg Temp and Rainfall % for each Month
• Average Wind and Wind Direction
• Stadium type - Open, Domed, Retractable Roof
• Field type - Grass, Turf