Alberto - I think you are "burying the lead". There is a new API that will do pagination, which seems like a good idea for longer tenure loans with many repayments and also especially for the performance reasons you mentioned. Now, you're asking for the value of Number of Transactions to return as a preference.
But, why would this not be a value to be set in configuration? And, how will this relate to the user interface (mifos web app) - would the web app show the number of pages? In terms of feedback, I think you should consider 10-15 as the starting point for the number of transactions to return. But, let's see if anyone has a strong opinion. thanks,