Sorry, I think I measured it incorrectly.
I have posted an updated pic. The smaller pipe is about 1ft in diameter and the larger pipe connecting to the filter box is about 4ft in diameter.
I hope I got the right one you were asking for measurements. Sorry about that!
Also, can you please clarify where the pre-filters cells that need to be cleaned are? When I open the filter cabinet there is only one washable blue cheap filter. I have also posted a picture of that.