For API calls without limit (ie. streaming), you can leave the parameter empty. From the UI, Looker supports a row limit of up to 5000 and recommends a column limit of 50 or less. If you need more than 5K rows, consider a csv or other type of download. Depending on your database (will it stream?), you can set it to 'unlimited results' or a custom row limit with a maximum of 100K rows.