If you can't know the width in advance, then your only possible answer would depend on staging your output in a temporary buffer of some kind. For small reports, just collecting the data and deferring output until the input is bounded would be simplest.
For large reports, an intermediate file may be required if the collected data exceeds reasonable memory bounds.
Once you have the data, then it is simple to post-process it into a report using the idiom printf("%*d", width, value) for each value.
Alternatively if the output channel permits random access, you could just go ahead and write a draft of the report that assumes a (short) default width, and seek back and edit it any time your width assumption is violated. This also assumes that you can pad the report lines outside that field in some innocuous way, or that you are willing to replace the output so far by a read-modify-write process and abandon the draft file.
But unless you can predict the correct width in advance, it will not be possible to do what you want without some form of two-pass algorithm.