These two fields may be confused and should be thoroughly validated and documented.
When using BDC File create, Batch Size should be blank. Likewise, when not using a file, File Split Interval should be blank. (It might be right to combine these two fields).
Batch Size should have warnings when the number is abnormally high for an online post (>500?). As this value gets large, more memory and roll back space is required and transactions or the system may fail due to constraints.