What design options can I go for when fitting a lot of content in a poster?

My boss has been wanting me to design a promotional poster that can be used for both 18x24in poster and a letter sized one page stand.

I have been having trouble designing a layout that would work for both letter size and poster size. There is so much content that I need to fit into one sheet. If I design a poster and would want to shrink it down to a letter sized page, all the contents would be ineligible. I had previously designed a letter sized page that would be able to expand to poster size page, but my boss wanted more of a poster feel to it, meaning more illustrative or with pictures... Is there an industry measure to tell if too much content will not be ideal for allowing both poster and letter sized page visible? Any good suggestions on layouts or templates that would allow lots of contents to be visible regardless whether it's the 18x24in poster or the letter sized page?

There is a total of 1.5 pages of contents if font is 9 in word doc. The poster will be in both Chinese and English whether on individual page or on the same page.

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