I don’t want to offend the original author, what do I do to make their message smaller but being polite about it?

Great question and concern. You never want to offend people because they maybe future employees/employers or relatives/loved ones.

The polite way to remove parts of the previous message without misleading the reader is to use the <snip> symbol. Either above the quote or below or in both places to indicate there was more.

Now if you want to do a word change, use these [ ] brackets to indicate word changes you made to the original. Like say if the author listed a bunch of REF character names and for the sake of space you want to swap out all their names to a general they. Then use “[They] where all in Krondor when James arrived, but where was Peter?” when quoting their original message. Be careful as some people might take offense to certain word changes and can call you the spelling/grammar police.

FAQ answers attributed to Raymond E. Feist are copyright by Raymond E. Feist.
It should also be born in mind that the answer given was only applicable on the date written, and to a specific question. You may find further, similar questions, in the FAQ.