I don’t think Postman is the best tool for that purpose - I could only spot a couple of examples online regarding pagination support in Postman and in most cases the recommendation is to do it in javascript/python or in general, using a more powerful tool.
Is it necessary to use the API for this report? If it’s just accounts you are after, maybe using search via UI is a better way?
In general, in order to get more than 250 results from a single endpoint, you need to query the endpoint more than once. A single query will only ever return 250 results.
In order to get more, you need to keep querying the endpoint until you have all the results you need.
If you insist on using Postman, you would need to query the endpoint once, download the results, then add an offset parameter to your call and keeping doing that until you have all the results. Bear in mind you’ll need to download each set of results and stitch them together later manually :).
This page explains pagination in IDN endpoints: Standard Collection Parameters | SailPoint Developer Community
Example:
First call:
/v3/accounts/?filters…&offset=0
Second call:
/v3/accounts/?filters…&offset=250
Third call:
/v3/accounts/?filters…&offset=500
And so on and so on.